Personally I really enjoy the Spire Corvettes, but I have to agree that they are rather under powered for a bonus ship, let alone a bonus starship. I use them anyway simply because if you start with them as a bonus ship, they are fairly ok early on. (More expensive than any other starship, but more fun IMO (I Like Modules)) However once you get past MKII the drop off really hard on usefulness. Where any other bonus ship or even basic starship I only have to invest the K to get the next mark up, with the corvettes I have to invest in all the modules as well. Now while that isn't as major of an issue for me with my "screw the AIP I have superweapons" gameplay, it becomes increasingly difficult to justify the shear cost of the corvettes each mark.
Now lets take units with some similar features. The mod forts have modules that the higher marks of you have to pay extra K to unlock, just like the corvettes. However, the forts can justify the cost in multiple ways. The forts are incredibly powerful for the K you spend, the extra K spent on higher mark modules gives you higher mark defenses for your planets, and you can even get powerful modules without spending any K due to them being champion unlocks. Of these three justifications, the corvettes only share one. The fact that unlocking higher mark modules gives you higher rank defenses as well. They don't have the usefulness and the power of the mod forts, and they can't get any specialized modules for free like the forts can.
Now, for these two issues I have a few ideas to fix them. The first, and most obvious, is to buff the corvettes. Keith already said he was thinking of buffing their range on the main gun, but I feel that they need a buff to their power, or their cap, as well. While they are supposed to be special things that you cant get as many as normal starships, they don't bring nearly as much as other starships can. If we have to keep the normal cap at 2, I would suggest a serious buff to their overall power. Otherwise bring the cap up to 3 or 4, and only give a moderate buff to their overall power.
Next bit would be to improve their integration with the FS campaign. The mod forts can get a good deal more useful, and you can get more of them, if you use the champion, but even without a champion you can still get one mod fort that is still damn good. I would say as you get further into the FS campaign you should get more out of the corvettes. (Yes this could make it feel like you are being railroaded into using them if you play FS, but you don't need them to finish and FS game and you can even beat an FS game without the FS fleet if you worked at it. This would just improve their usefulness if you had them during an FS campaign.) Perhaps they can get special modules that you can only unlock while playing the FS. Or, even more awesome IMO, you could get larger Human/Spire hybrid ships if you have the corvettes during an FS game. The other ships wouldn't need to be much more powerful than the corvettes themselves (maybe 20%-30% stronger) but simply able to add more modules. In this case, I would decrease the cap by one each time. (IE, if corvettes were bumped to a 4 cap then the next two would be a cap of 2 and 3 respectively) The last idea, which someone already suggested and I am shamelessly reposting, would be to increase the cap of corvettes as you get further into the FS campaign.
(I used the mod forts as thecomparison to the corvettes because while they are radically different units, they share a common theme. The mod forts are human fortresses integrating the shadow vessel's modular tech, and the corvettes are simply human starship design being integrated with Spire technology, similar to, but on a larger scale, the Riot Starships)
