By analogy, this topic reminds me of website's hit counter -- easy to criticize for not being reliable but might provide order-of-magnitude sort of information. Some of these metrics have serious limitations but taken together, may provide some useful information.

For my own work area, I certainly won't switch languages based on current popularity (I wasn't a fan of FORTRAN on VMS even when that was "cool") but I do feel a degree of responsibility when writing code for others to use routinely. I want my code to be useful/maintainable after I've moved on to other projects.

With this in mind, I recently wrote a significant application in visual basic for our biologists. One reason for this choice was that I thought I would be making it easier for the next programmer. I see now that my nice gesture was probably in vain. I don't mean that I would have switched to Java but it probably would have been better (in my case) to have just stuck with Perl.