Exporting of a key has a problem, doesn't it: the key is still in the source wallet. There's no way to remove it, is there? Even if there is, it's easy to screw up and people will wonder where their money went.
I do not seet it as a problem, so what if I have the same private key in many wallets? Actually for me that is the case, because I store my wallets in many places on the internet. Encrypted of course.
That's backup-copies. I have those, too.
But say I give a privkey to a friend as birthday present. He imports it at some point and I still have it in my wallet. Now there are two active bitcoin clients showing the balance of that key. If one of the two spends the money, it will disappear from the other client, also.
So my friend or I will wonder "where the hell my/his money went"