As I look back on my career, there have been things that I learned early on that I still am able to leverage today (i.e. bash, linux, vim, java). There are a slew of other technologies that didn't last (i.e. DCOM, Corba, J++, J2EE). If I'm going to invest my time in learning, I'd like to realize some return on time. Granted there are cool experimental technologies that I learn with the realization that they may not be around in a couple years, but I can plan/architect around that.