Yes, throw away everything you know every now and then, instead of building knowledge on top of knowledge.
That will get you far, right?
Who says you have to throw away everything you know? I don't magically stop knowing C++ when running around in Ruby or SQL.
I like relearning. I don't have much use for re-learning... I'd rather learn to do something new. The "to accomplish the same things, only on Windows instead" is the crucial qualifier. I'd rather spend my time, I don't know - learning to configure nginx or doing Haskell tutorials than learn someone's proprietary shell, given that shells aren't particularly exciting and I already know enough about the shells that every other platform offers to be satisifed.
Learning new stuff is cool. Relearning same old stuff in different clothing because someone decided to do something differently is not cool.
PS is substantially different enough that I don't mind, though.