I dunno. i already feel fairly railroaded when I am doing FS.. because no ships are useful at all.
It depends on how deep into the game you get. Besides that, the main fleet usually ends up on defense to deal with the exos, at least in my experience with FS. If they're not needed, they're a secondary team taking on secondary objectives.
This has been a problem for me for quite a while - I simply do not like any of the bonus ships. The only ships i could see being useful at all are munitions boosters, and even then the flagship fills mostly the same role
Munitions boosters boost to a huge level of difference compared to the flagships, so there's value there.
In general though, I agree. The fleet eventually becomes irrelevant. That's kind of the point of that campaign. The Spire patted the AI on the head when it showed up and blew them out of the stars. Then the AI got serious. If you don't want the superships, the ones that make the human fleet look like the tinkertoys they are, don't play FS.
The Spire Corvette itself strikes a nice balance to me, it's a workhorse in my current AAR game. It's got some reasonable firepower, and can mount sniper cannons for some reach out and touch someone. For anti-FF work they're 2x as fast as a siege and can mount siege cannons, duals if you really need the firepower. From a starship perspective, if you invest in the module upgrades, they're on par with the 50% better than the equivalent mark of starship that bonus fleetships get.
I'm not convinced in any way there's a problem with the corvettes as they stand, though I wouldn't mind that beam-upgrade to get them out of knife-fight range.