While a lot of us already know all this, I think it's easy to overlook some of the raw facts about Command Centres - both in terms of using them yourself on your own bases, and being "fearful" of an enemy base with a lot of command centres. There's important facts about Command Centres that underlie all of this:

* Command centres only affect the base owner.
* Command centres only affect fleet (not turrets)
* Command centres increase the "attack power" of the fleet (whether in defense, or if the base owner uses the fleet at a base to attack).
* Command centres do not affect armour at all.

If You Get Hit?:
On the (hopefully rare) ocassion that you get hit, and the enemy has a huge amount of recyclers on your base that are going to be there for a while (a couple of hours at least), and you have a load of corvettes/fighters/bombers (potentially even destroyers) handy in relatively close range that you can sneak in safely to hit the recyclers with... command centres dramatically increase your recycler pwning power. However, these times are incredibly rare. When it does happen, it's awesome. Personally though I've never experienced this at any one of my bases, and I work towards making an enemy not ever want to hit my bases in the first place.
I would however consider them for this reason at a base that is more likely to be hit for tactical/war reasons - in other words a high jumpgate base - as these have a much higher likelihood of being hit with no thought on "profit", as high jumpgates just need to be taken anyway.

If You Get Occupied?:
On the (hopefully even rarer) ocassion you not just get hit, but the enemy makes a conscious effort to occupy you (ie placing HC's/dreads on your base, topping them up with fresh fleet supplies whenever you take the occupiers out), command centres allow you to remove the occupying force much quicker, much cheaper, and creating less debris for the enemy to collect. This can make the enemy really hate occupying you. However it must be said that the majority of players at mid to high end playing very rarely get occupied "at length" in this manner (especially if your base is set up so nobody wants to hit you), and when it does happen it's more than likely you or a guild-mate has proper mobile fleet to remove the occupation permanently. Once your bases are bad targets, and you are in an area well covered by guild-mates, this fact should not be a consideration for command centres, certainly not large amounts of them - the energy/area/population might be better put to use elsewhere. For bases in risky galaxies though, that might well be occupied a lot... well firstly you shouldn't have a base there But in these cases it might be prudent to have high level CC's in preperation/as a deterrant to occupiers.

For Your Fleet Base?:
This is both true, and not true. If the enemy is stupid or does not have enough bombers/heavy bombers/fighters to take you out, then yes, high CC's mean the enemy will spend an absolute fortune hitting you, reducing their overall profit and perhaps reducing the likelihood they try to hit you. However there's something very important that a lot of people forget to take in to consideration here;
Command centres do not increase armour!
Why is this important? Because a resourceful and clever enemy will hit you in waves of fighters, bombers, and heavy bombers - using the bare minimum necessary to kill your fleet. This means your attack power is irrelevant - they are using just enough to wipe out your armour and the units will die whether you have 0 CC's or 20 CC's. People must also bear the reverse in mind when attacking an enemy base with huge fleet and high CC's - spend some time calculating how to hit it with waves of (mixed or individual) fighter, bomber and heavy bomber, and you should find that the CC's make little to no difference.

I'll give an example. Let's say an enemy has 15/15 P-Rings, 10/10 P-Shield, 10/10 Ion Turret, 20 Command Centres, and a fleet consisting of perhaps 75,000 fighters, 20,000 bombers, 15,000 corvettes, 20,000 recyclers, 8,000 destroyers, 9,000 cruisers, 2,000 heavy cruiser, 200 Fleet Carrier, 30 Battleship and 5 Dreadnought. That's 5,200,000 worth of fleet.

Now to take it out, you could go a few ways, but to keep it simple let's start with a fighter drop - 204,497 fighters removes everything below cruisers. 1,022,485 losses for us, 1,848,400 losses for them. Next we drop 64,000 heavy bombers. 1,920,000 losses to us, just over 3,196,000 losses to them. All that remains are battleships and above. We do a final hit with 8,500 cruiser and just 10,000 fighter meat-shield, which takes the base - killing all remaining defenses and fleet. 185,000 losses to us, 160,000 losses to them. Our total losses are 3,127,485, theirs is 5,200,000, with around 4,430,064 debris made. Not a bad profit overall against a base with 20 CC's.

But what happens if we do exactly the same attacks, but there are 0 CC's?

The first (fighter) wave happens exactly the same - identical losses both sides, no difference in costs or debris.
The second (heavy bomber) wave happens exactly the same - identical losses both sides.
The third wave (cruisers & fighters) is the only time the CC's make a difference - why? Because we are not sending "just enough fleet to kill" that also dies by itself as we do with fighter/bomber drops. We are sending a fleet that is meant to "survive", which means more damage incoming is bad. Here with 0 CC's we lose 149,295. Their losses are the same. 20k less debris, with 35k less losses. A difference of 15k.

20 CC's compared to 0 CC's in this instance made a difference of only 15,000 credits


Summary / Conclusion:
Command Centres certainly have their uses. For yourself on your own bases, it helps you on bases in dangerous areas where you are likely to be hit quite often or occupied quite often. If it's a fleet base, you can ward off less tactical enemies with high CC's, or cost less tactical enemies a lot more fleet. If it's a very high jumpgate base and your guild often uses it as an LZ for your galaxy, then actually I really recommend a high amount of CC's, as LZ's are common targets for enemy - either because you get inactive guildmates leaving their fleet at your base attracing enemies, or because the enemy wants to take your jumpgate out of the picture, and there's a good likelihood every now and then that they leave huge amounts of recyclers at your doorstep because of this.

I highly recommend combining your fleet base as a high jumpgate/LZ base for your guild as a whole, and working on a high amount of CC's on this one base. High CC's really do benefit on a fleet/jumpgate base. But you should only need one such base...

However do not over-estimate them, and do not go mad with them on every single base you own. 20 CC's compared to 0 CC's can make a massive difference to the cost of a base - what do two orbital bases and two multi-level platforms cost you on your high-end bases, plus whatever energy costs there are, plus the cost of the 20 CC's themselves? For me, counting in the energy costs too, this is around 450,000 credits worth of stuff on my well developed bases, give or take a bit. That's a massive cost for something that is likely going to make next to no difference to my base. I personally keep 10 on each of my "regular" bases, and I am tempted to lower this to 5, possibly even 0 on all but a couple of select bases in key locations. Don't forget to keep some command centres though, you need 1 command center per occupation you wish to hold, regardless of which base you have the command centres on (be it 1 on each base, or 20 on 1 base)

And finally - do not be too scared of enemy CC's. There's a basic rule: If you can take most of their fleet out with fighter drops and bomber/heavy bomber drops, their CC's mean literally nothing, they may as well not be there. If however you need to hit the majority of their fleet with your own fleets (cruisers, heavy cruisers, frigate fleets, whatever), then the CC's are going to be costly to you.