There has to be some kind of taxes or other kinds of support (if not money) from member worlds for Starfleet. Otherwise why doesn't everyone just kick back and say "let the stupid hew-mons handle it." Hmm, waitaminute... 
The Dominion is accepting membership applications!
Now that would be a fun plotline. Forget the shootin' war, let's have the Feds and Doms actually in ideological competition for expansion. The Dominion offers a lot of benefits - total protection from outsiders and no domestic interference as long as the population behaves. A very pro-business ethos, too! No Dominion member world needs to send their people as cannon-fodder, since the Doms grow their own. The Feds would be hard pressed to counter that offer.

What about a benign military fascist regime?
The Dominion is accepting membership applications!

Now that would be a fun plotline. Forget the shootin' war, let's have the Feds and Doms actually in ideological competition for expansion. The Dominion offers a lot of benefits - total protection from outsiders and no domestic interference as long as the population behaves. A very pro-business ethos, too! No Dominion member world needs to send their people as cannon-fodder, since the Doms grow their own. The Feds would be hard pressed to counter that offer.