until you destroy 99% of all the wallets and then you can't exchange them for anything useful.
I'll wait for bitcoin 2.0 :)
There already is bitcoin 2.0
tl;dr testnet
plus - there is slight chance someone will generate same private/public key combo and suddenly "find" "lost" coins =)
Except bitcoin has no way of marking a coin as "lost" so if you did generate that combo, everyone would be in agreement that it already belongs to a lost wallet.