Not true. You 'dump' the knowledge that you don't use. It's also got quite a lot to do with stuff being in our long term memory and short term memory (WWW will be able to expand on this one, he knows about this sort of stuff).
The proof: ask a 54 year old airline captain something that was in the ATPL syllabus in 1965 (such as Astral Navigation?) and he/she probably won't be able to tell you very much about it. However ask him/her what Vso is of the aircraft that they fly day in, day out and they will be able to say without hesitation. It's all about repetition.