I agree that mobility is important in the career of most academics. Indeed, most of us have studied and worked at several institutions.
....the musical chairs that us academics play in our careers serves to disseminate our knowledge.
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?