The Importance of Being Mobile

A comment on this post reads:

....the musical chairs that us academics play in our careers serves to disseminate our knowledge.

I agree that mobility is important in the career of most academics. Indeed, most of us have studied and worked at several institutions.

I was reminded of this comment yesterday, when I was asked to fill in a EU questionnaire on the mobility of researchers. One of the multiple-choice questions on the form read: "How often should a researcher move at different stages in her/his career?" (I was asked to answer this question since I claimed that mobility is important in the career of a researcher.) For instance, how often should one move over a four-year period at the early stages of one's career? I assumed that this question was referring to the first four years after one's PhD, and my answer off the top of my head was 1-2 times. (What I really meant was twice, but I thought 3-5 times was too much; the rationale being that one should be mobile at that crucial time in one's career, but that being overly mobile might cause too much overhead---especially if this involves changing countries. Later I looked back at my movements in the period 1991-1994 and realized that I actually moved 5 times myself.)

What is your opinion? Is mobility important at all stages of one's career? And how often should a researcher be mobile during the first four years of one's career?