Yeah, the United Federation of Planets. Not the United Coalition of Planets, or the Economic Community of Planets, or the Planetary Alliance Treaty Organization. A federation, not a confederation.
Firstly, permitting federal subunits to possess and organize military force brings in 1)the danger of secession, and 2)the danger that resources meant to be devoted to the common defense by a federal subunit, perhaps developed partly from federal funds, might not be used in the common defense at all.
Which, again, is consistent with anything but the United Federation of Planets as depicted in Star Trek. I am not prepared to visualize this political entity as being composed of shifty, self-interested quasi-nationalists who would just as soon screw over their fellow members whenever it's convenient for them to do so. As I said, if their governments were THAT backwards, they wouldn't have joined in the first place.
Nor am I prepared to posit a Federation of Planets that exists as a de facto Earth Empire, composed of hundreds of alien worlds that have been politely subjugated for their own good. The primary moral imperative of the Federation is freedom and self determination, and it would duplicitous to the extreme for these values to be upheld by a culture that begrudges those rights of their own members.
The other thing is that we don't know for sure how these ships are paid for; the Human-dominated Starfleet could well be paid for entirely through Earth resources. Same for the Andorian/Vulcan/Tellarite fleets. And even more to consider that in some conflicts--the Cardassian Wars, for instance--it's entirely possible that the whole of the Federation might force one of their members (namely Earth) into a fairly one-sided settlement just because none of the other members had any stake in the outcome (it was only HUMAN colonies in dispute with the Cardassians) and weren't willing to commit much of their resources to the war effort. Federal Control works both ways.
Secondly, self-determination's great and all, but signing on to the Federation should mean accepting its values--permanently.
Values and cultures can and do change. I don't see the Federation using military force to prevent some of its members from seceding. Obviously, they would do everything in their power to encourage more rational elements to prevail, but if popular sentiment favors secession, there is nothing they could do about it that wouldn't be in strict violation of their fundamental principles.
I can't think of a single secessionary or irredentist struggle that has occurred within the borders of a democracy* that has ever, ever been a good thing.
The American Revolution, for one.
Plus the fact that all of these members joined the Federation voluntarily, not by accident of geography and national heritage. To say to them "You're in, you can't leave ever again" would be to tacitly admit that they have been conquered in by a relatively non-violent empire. This would be all the more gross in the case of, for example, Bajor, where their entry into the Federation was literally brought about by the fact that the Bajoran religion latched onto a Starfleet officer as its primary religious figure (and this same officer had standing orders to do "everything short of violating the prime directive" to bring Bajor into the Federation; this is all well and good, so long as the Kai is still in power).
Self-determination is not at all necessarily good.
I'm sure you believe that, but the Federation does not.
I don't want to live in the UFP if it's okay for someone to, say, stone their wives to death for infidelity, as long as they're from the planet Rajmadorian, and they're sincerely acting from their cultural imperatives and within their local laws.
The UFP already has that rule whether you want to live in it or not. What you are overlooking is that, under those circumstances, Rajmadorian would not be allowed to join the Federation until it changed some of those laws.
OTOH, it's legal on Vulcan for competing males to fight to the death over a woman. Rajmadorian might well get a pass...
I'm even wary about accepting a UFP that outlaws whatever barbaric practice we want to use as an example, but permits the barbarians to retain significant military force that could be used to defend that barbaric practice in the event the Federation came to enforce its abolition.
Again: the Federation doesn't impose its morals on its members. Quite the opposite, in fact: it simply excludes from membership people who refuse to live up to Federation morals (as it did with the Maquis). I would expect that rather than start a war to impose order on those worlds (as the Dominion would) the Federation Council would probably start with sanctions and trade limits, with removal from the Federation as a last resort. If you can't live by our rules, you can't be a member.
I want to live in a Federation, not in a collection of vaguely like-minded, collective-security-conscious, socially-unpleasant aliens.
The Federation IS a collection of vaguely like-minded, collective-security-conscious, socially unpleasant aliens.
I mean, last time I checked, the Tellarites and the Andorians were still members. And the Vulcans have been described as "socially unpleasant" by pretty much everyone in the galaxy.