I think collaborations bring out the best in flash art. Great, focused programmers working with awesome focused artists make some of the best games.
A good animated collab brings many styles to the viewer in a short amount of time. Getting together a lot of people under one idea is easier on the viewer in my opinion. He or she isn't stuck inside the imagination of one person. Instead, they get to take in the ideas of many people all at once.
Collabs are also great for exposure in the flash community. If your part is of note alongside a part from a more famous artist, it gives you a great deal of credit.
Unfortunately though, I think a lot of artists rely on collabs much too heavily. I see plenty of users on NG that have sky high Batting Averages with none of their solo projects even effecting it.