I hate to keep tapping BOTF, but I think that game more than any other out there handled the idea of second string races better than any other.
Some minor races didn't build ships, others did. I ALWAYS put effort into assimilating the races that did through peaceful means. Even though compared to high tech ships I could build towards the middle game, their fleets weren't uberpowerful, they DID give you a nice edge in the early game when my own fleets were gimped by the need to do the colony rush. Some races built light raiders, other built large cruisers. One race, the Yridians, made cloaking scout ships. If I could get them on board early, they could wreck shop with anyone that stepped up.
None of the minor races colonized, but they did put spectacular effort into building up their homeworlds. If you took your time courting them, they could develop their worlds into spectacular research, financial and manufacturing bases that took a LOT of pressure of your own homeworld. This is something akin to what the GalCiv races do now.
All the races had a unique race structure that provided a bonus when they were brought into your empire and allowed to develop. Some offered research bonuses, some offered academies where your ship crews could train outside of combat, some offered food, manufacturing and financial support. Each was unique that made each race something special, and some races EXTREMELY desirable. There was some overlap, but only a few cases (the games where I contacted both the Vulcans and Trill before anyone else were damned near guaranteed wins in the research game, even on Impossible difficulty), but the fact that each race had something to offer your empire through assimilation made each something you treasured when you got ahold of it, and something you fought tooth and nail to keep intact. There were times when the Borg showed up, I'd sacrifice my Federation colonies for the chance to put a fleet together in the defense of a minor race homeworld.
That's kinda what I'd like to see the minors here become. Not something individually uberpowerful, but something that when they were assimilated, were something that made you even better than simply adding another star to your individual constellation of influence.