Well, there's also the problem of "Requires X years of Y experience" jobs... if you are (e.g.), a Ruby person looking to move ahead, you want to increment X years in Ruby so your resume isn't trashed by the HR screener or the automated resume scanning program.

So I disagree: it's a career strategy that is optimized for HR screeners and resume scanners, which most bigcos seem to use.

(I personally prefer generalist positions but I don't see those advertised outside of startups).