First, there are lots of reasons to join a world and equally as many, if not more, reasons to leave it. One of the worlds I was in went public to fill, and didn't tell anyone who was joining that the previous season they had voted on starting a prospect cap. Now I am not necessarily against a cap, but I don't like when leagues go public without a blog that has the rules posted in advance. It turned me off, and the commish's attitude about it was so off putting that left after one year. Another world I joined I didn't look closely enough before I took the team and there were 2 guys in AAA making 4 mil + for three more seasons. AND they had a 38 yr old Hall of Fame calibur player in steep decline (66 ovr) that is due 9 mil for three more. Wasting 20 million a year for 3 seasons is a problem considering the team has few prospects to speak of. Now I might choose to keep that team, but if I don't I will take a new team after I leave. While I have it though I won't make the situation worse by not signing my first round picks, or signing more absurd contracts. Every owner will leave their team eventually. I beleive that quality of ownership is much more important that duration. Leaving teams an utter disaster area is what makes worlds hard to fill.
To address your commitment comments Mike, I have read some of your stuff in the forums. I assume you don't go public to fill your worlds. If you are a private league, the commish has every right to ask before they admit a member whether they plan to stay long term. If that person says yes, and they then bail that is something to be upset about. If you go public to fill your world, or you just give a guy the password when they sitemail no questions asked, there is no commitment beyond that one season. People can believe there is if they want, but in reality there just isn't. Easy come easy go they say. Being upset about a guy or two leaving to join a new world when there are 4-8 openings every year is silly.