Why I think it is a good thing the Federation, Romulans, and Cardassians aren't friends

So I was thinking about Star Trek this morning (ok I'm usually thinking about Star Trek but this was a very specific thought).

I'd be interested to know if this was a conscious decision, but I find it interesting that the races that probably have the most in common with humans in terms of character traits are probably the Romulans and the Cardassians. However, these are guys are cast as the antagonists in the Star Trek universe.

The strength of the Federation is embodied by lots of different races very different to humans working together and bringing their unique qualities to bear, even where those races may seem "inferior on paper". Indeed the only super power the Federation seems to get on with is the Klingon Empire, a race completely different from them and who both the Romulans and Cardassians tend to treat with contempt. Yet it is these alliances that continually allow the Federation to succeed, even in the most dire of circumstances.

A lot is made of diversity in Star Trek and rightly so, but I guess I've never considered how it regularly considers the weakness of lack of diversity (as an aside it is the issue I have with the Mirror Universe - although implied that not everything is as strong as it looks, having this seemingly all powerful Terran Empire breaks the rules. It should be mighty, yes, but fragile to new situations. I've not watched all mirror universe episodes though so this could be more evident than I'm giving it credit for).

Anyway my point is that Star Trek is often praised for its progressiveness on an individual/personal level, but if you step back, it really does a great job of showing that if you don't embrace diversity at societal level and treat every possible sort of difference as an asset that makes the whole stronger, you're at a distinct disadvantage in dealing with those exceptional crises that challenge your status quo.