Mark Shuttleworth has written a nice little blog post about the tools we learn through life and how we discard old tools and learn new ones.
I personally find this to be very true in my life.
I think Mark’s onto something. Changing toolsets often is definitely useful. Even though I couldn’t write full programs for you in Perl nowadays, what I do know about it (its limitations, capabilities) is definitely good enough to see when it may be the best choice for the job.
As for academic tools — very true. A lot of techniques I learned in e.g. Discrete Math, Linear Algebra were in one ear and out the other. Alas, I think the main point is to learn them once and then be able to Wikipedia them later, when needed
That said, stuff I learned in my algorithms and data structures and operating systems courses have stayed with me. I think some of that stuff is just essential.