Really? So the United States and the United Kingdom should not have allied themselves with the Soviet Union during World War II -- even thought they may never have been able to defeat Nazi Germany without the Soviet alliance?
I'm not saying that allying yourself with an evil government is ever a good thing, or that it's ever just easy, or that it won't contain risks and costs and consequences. I'm not saying it will never come back and bite you in the ass. I'm not saying that it's always, or even usually, what liberal democracies ought to do.
What I AM saying is this:
1) Sometimes, in especially dire situations, it is necessary. The US and UK needed the USSR to defeat the Nazis; the Federation needed the Klingons and the Romulans to defeat the Dominion.
2) When liberal democracies DO form alliances or agreements with evil governments, sometimes the result can be that their good "corrupts" that evil government's nature. Does anyone really expect the regime in China to survive as the world becomes more and more interconnected and liberalized? I don't.
This is true. The UFP doesn't really "care" about the internal government (see: Prime Directive). It wants peacefully coexistence. For membership within the UFP, however, we know that the requirements are strict.
I still see the UFP as wanting Qo'nos and Cardassia to take their "rightful place" at the Council, though. Both seem on likely paths towards admission: the Klingon Empire was dieing and Cardassia was dead. After any nation collapses, it can be rebuilt according to the aid-givers standards. The UFP being the most powerful nation in the quadrant, is the likely Superpower to give that aid. With a Kitomer-esque peace treaty OBVIOUSLY on the horizon with Romulus, it seems likely that the only two signifigant concerns for the UFP would be from a resurgent Dominion (unlikely because it's obvious the UFP was not going to make the mistake of being caught with its guard down again and the Dominion pushed back through the wormhole) and the Borg.