voodoo,
I agree with nearly everything you're saying. Except, we have seen several players who have been drafted only because they were committed to a JUCO. In other words they would not have been drafted at all had they been insistant on attending a 4 year school. For these players, waiting until after the draft to commit to a JUCO could get them undrafted.
While it is rare, we know of one D&F who signed after his first year at a JUCO (with the club that drafted him) for late 1st round money.
So, there can be an advantage in some cases in committing to a JUCO before the draft, and specifically because of the draft. I suppose you already know that and are speaking more in terms of leverage.
Not much leverage involved in JUCO signings.