Block layer discard requests

Umm, perhaps I'm naive, but I thought (hoped) that wear-leveling already 
re-used existing blocks, at least hardware based ones, or is that too
expensive? It's seems the only sane way to have long term guaranteed
reliability, by moving content on little written blocks to elsewhere and
using those blocks for frequent writes too. Sure, writing n blocks means 
you need to read n blocks and write 2n blocks, but all blocks would be 
written the same number of times in the end. This shuffling around doesn't
have to happen all the time though.