One perspective that no one has touched on yet is this: if I were a beginning job candidate, with a Ph. D. on which the ink was still fresh, all this talk about job-hopping junior professors would be making me nervous. I'd wonder how I could ever compete with people who already have a tenure-track job and are looking to move up. I can't say anything useful about the junior/senior split, but I can talk a little about this issue. (These are my personal opinions and observations; if, God forbid, anyone figures out who I am or where I work, they should not be taken as either exemplary or representative of the opinions there.)
First of all, things do even out when you have a pool of candidates. Let's say that you have 200 qualified candidates for an average search (I've heard of some pools of 700, but I don't think that's usual). Of those, extra materials will be requested from some--a shorter list, and maybe 10-12 will be interviewed at MLA. (Some schools interview 30 or more.) Of those, 2 will be invited to campus, and 1 will get the job.
In that mix, when considering who'll be invited for MLA interviews, there'll probably be some new-minted Ph.D. degree holders, maybe some advanced ABDs, some new assistant professors looking to move, and maybe some advanced assistants or associates also looking to move. Assume that the letters, research agendas, and so on make those in this list (maybe 30-50 people) seem like good prospects. Each category, though, has advantages and disadvantages:
A committee can't second-guess why candidates apply for a job, or why they want to move, or anything like that. It can only try to choose people who seem to be the best fit. My point is that candidates of all levels can be that "best fit."
[Updated to add: There's an index of all the posts on the junior/senior divide at Prone to Laughter.]