I use it, i like to sneak a siege frigate to bombard or a capital ship to get the damage increase, I always use it if I researched it. As far as I've seen it works after the first second of the first bombardment attack you made till the moment the ship leaves the grav well, your ships will have the increase for long, not sure how long though. Its not supposed to increaase damage from siege frigate bombardment but it does... Or maybe I am wrong.

It applies a real bonus but you're right it doesn't show up on the infocards. And yes if you have a ship actively bombing your ships get it, as soon as it stops you lose the bonus.

I mean if you can get it, if you can spare a capitalship (like the Akkan) that doesn't need to be to be actively using abilities in combat, by all means take advantage of it. I personally like the one that gives you credits for bombing more but if you're planing on going on the offensive it can be worthwhile to get both.

Siege frigates only tend to work against the AI, as human players will try to take them out with Corvettes or fighters ASAP, whether they know you have those techs or not. They're just too fragile to use in any sort of moderately contested planet. I prefer to use them to raid totally undefended planets for the credit income, if the enemy sends reinforcements, you still earned some cash, and if you take the planet, even better!