While there is only one type of "watch-only" address on the technical side, there's two different
use-cases for it:
 - addresses you really only want to watch (and that you don't even have the priv key for)
 - addresses, whose key you have, but didn't want to be saved (even encrypted) on the server.

If you use a watch-only address in a transaction, it will prompt you for the priv key (seemingly
to support the second use case).

It might be nice to be able to tell blockchain.info which of the two types it is, when
importing a watch-only address. Addresses of first type then also shouldn't add to
the total funds.