Except that:
- Capital ships are over 5 times as durable, depending on the type and level.
- Capital ships can contribute to fleet control of the planet, which siege frigates can not do.
- One capital ship replaces 4 siege frigates in fleet supply, not 5.
- Some ship powers flatten planets better than going for pure siege ships. Raze planet and drain planet are cruelly powerful to planets, plus there's a few more.
- Capital ships cost less for their benefits overall.
But that is not the subject. It's actually not about Cap Ships vs siege frigates but about bombing a planet or not. (and the comparison was not made regarding fleet supply points but regarding resource costs). And if you want to bomb a planet the only useful option is a handful of siege frigates. In my opinion you SHOULD bomb each planet because:
1. only taking out fleet and orbital structures cripples defenders economy = half effect
2. taking out the planet and taking it for yourself cripples defenders economy and strengthens your own = full effect
You simple give away half the benefits of attacking an empire if you do not take over control of the planets you have attacked.
Since destroying a planet takes soo long, it's better to wipe clean the orbit of one planet, and just move to another one
As I said this is only a question of how many siege frigates you use.
Usually in this situation I am a long way off from a re-supply area so my fleet is basically on its own. Any chance to supplement my fleet is most likely going to come in the form of a sister fleet, and not just replacing losses
And this is another vital pro for siege frigates. If you destroy and conquer the planets of your enemy you have the chance to build frigate factories near the front line and get a much shorter way for supplies to rebuild your fleets.