Do you really have a use for a local copy of the blockchain?
If you don't, then I'm sure you'd find it much more convenient to use a Multibit or Electrum, you won't have to store 20 GB of data, and wait days to start using the client.
And this is the problem. Eventually nobody except large centralized servers will store the blockchain. This will lead to issues with security if the owners of those large centralized servers experience "issues" like MtGox did.