While I won't deny that all bonus ships are USEABLE, I really don't think that's any measure of balance or good game design.
Choosing between a Raider and a Space Plane is like choosing between a slingshot and a bazooka, and I could give you tons of more example just like this. That isn't to say that the Raiders are useless, or don't buff the DPS or my fleet, or anything of that nature (I mean when it comes to fleetballing, why does it really matter WHAT you have, as long as it moves and shoots?).
However, I agree with RCIX, we should aim for a more ideal balance between the 80+ bonus ships we have. Some people may be like, "Well dude, if you think a bonus ship sucks, just don't freaking use it."
Good point. That's great. Except the AI may end up using it. In fact, how difficult the game is depends heavily on what bonus ships the AI unlocks. If the AI unlocks Eye Bots, Railclusters, Spire Stealth Battleships, and Rams (i.e., my current game)...my god, you are in for a world of hurt. Whereas if it unlocks Raiders, Minipods, Microfighters, and Chameleons, well...massacre probably isn't a strong enough term. The bonus ships the AI gets can literally raise or lower the difficulty by an entire level. If I'm playing difficulty 8 and they get the right unlocks, I may as well be playing difficulty 9.
If I'm playing difficulty 8 and they get the wrong unlocks, I may as well be playing 7. And everything inbetween.
Personally I think every ship the AI has access to should be a threat. Of course the humans can just choose the best ones, and have no reason NOT to. But the quality of the game is severely dampened by the fact that the AI is often placed with such underpowered units compared to what you are using. Which is why I wish we could get a bonus ship omission file added, or at the very least admit that this is a problem.