Caps really only shine in comparison to siege frigates. While siege frigates deliver more pure, unalloyed bombardment strength compared to caps, caps are much, much tougher, more flexible, and hard-hitting to enemy ships and structures.
Here's a per-population comparison of a Krosov Siege Frigate to the Kol Battleship:
Korsov Siege Frigate
30 hull
15 shield
0.43 DPS
1.33 Bombard
Kol Battleship
60 hull
25 shield
1.03 DPS
0.86 Bombard
And of course this is only a level 1 Kol. As the Kol increases in level, it will get more hull and shields, and of course, more powerful abilities. Now this added power is, of course, expensive in the short run, as you'll need to spend credits to increase your Capital support. However, I submit that in the long term, the Caps are an excellent deal, since they deliver a lot more flexibility and power for the same fleet supply, and your fleet supply limit is a constant drain on your resources, which will accrue over time and increase as the size of your empire grows.
All this said, bombardment is a luxury you can only really engage in when you've got uncontested control of the gravity well, so overbuying caps when your fleet is overmatched is not a wise decision.