Conventions are basically big parties that combine entertainment with education, with ancillary features such as masquerades/cosplay competitions and dealer's rooms added on to suit the particular convention. Each of the local conventions reflects the tastes and preferences of the people running it, which in turn reflect the tastes and preferences of the people they hope to attract. This post about room parties at Minicon 40 brings up an interesting measure of what, for lack of a better word, we can call the "social health" of the convention and its attractiveness for fans who aren't regulars.
The thing about room parties is that they serve both as a means of advertising for fan clubs who are either promoting their own conventions or themselves; in either case, they're looking for people who share their interests. Room parties can also be considered part of the entertainment at the convention; I think Captain Kirk's House of Ill Repute, the House of Toast, the DDR Ring, and Dystopia at last year's CONvergence fall into that category. People like to throw parties, and there's a certain amount of egoboo that comes with throwing a popular one. If they're open parties, they contribute to a convention by attracting people who otherwise might feel out of place in the convention mainstream, and that helps those people enjoy themselves, which in turn encourages them to think about returning to the convention next year.
This being the case, most conventions promote room parties and try to locate them in the same area of the hotel so that their members can sample the various room parties and get a taste of what the various groups have to offer. If a convention is popular, it'll attract more room parties, and a healthy cycle can begin where the increasing number of parties attracts more people who in turn tell their friends, and...you can see where this is going.
Can there be such a thing as too many parties? If that's all you're doing at your convention, probably so, but I don't know of any that do that. There was some concern back in the day about "frat boys" showing up at Minicon to get drunk and ogle the femmefans, but that doesn't seem to have carried over to CONvergence, which has a dry consuite anyway.
In general, the Twin Cities have a nice assortment of conventions, even if they do tend to be mostly crowded into March and April. Marscon kicks things off at the beginning of March, Anime Detour follows at the end of the month, then Minicon comes along on Easter weekend. Then follows a two-month drought until CONvergence in July, Diversicon in August, and another hiatus in September until Arcana and Consume/Relaxicon in October. I'm leaving out the gaming and comic book conventions, because they don't really fit cleanly into the same class of conventions I'm talking about here. I personally think that the local scene could comfortably accommodate a few more small conventions, perhaps on Memorial Day or sometime in September, but I have enough trouble running the convention I'm responsible for, not to mention helping out at Diversicon and Arcana.