I don't see how that makes sense, because the Federation is just that, a confederation of sovereign nations cooperating for mutual benefit and defense.
Erm.... No. If it was, it would be called the "United Confederation of Planets." That it is called the "Federation" means it is exactly that -- a federal state.
Ben Sisko said onscreen in "Battle Lines" that the Federation is "made up of over a hundred planets who have allied themselves for mutual scientific, cultural and defensive benefits." It is a federal republic -- I didn't mean to imply otherwise -- but it's one where the individual worlds are still largely autonomous.
After all, we're talking about whole planets
, each one of which would have billions of citizens, thousands of regional and cultural subdivisions, etc. Look at how difficult it is to govern even one large country on the surface of a single planet. Imagine how much harder it would be to do that for an entire planet. Now imagine how exponentially harder it would be to do that for a hundred and fifty planets at once, especially when they're separated by vast distances in space and whose populations belong to whole different species with different values and priorities.
Trying to regulate all that with a single, monolithic government would be like herding cats. It's totally beyond the realm of practicality. The only way a system like that can work at all is if the individual worlds take the bulk of the responsibility for their own governance, economy, and internal affairs, with the federal government limiting itself to matters of mutual concern such as interstellar trade and diplomacy, overall defense, and so forth. Maybe a better analogy would be the US under the Articles of Confederation -- a federal union, but a loose one in which the states retain much autonomy. Nothing more is practical on an interstellar scale.