It doesn't matter. The purpose of the hash isn't to uniquely identify users, it's to narrow the list of users that need to be sent down to the phone. If Path could send their entire user database to the phone, they wouldn't need to send the contacts to their server.
Replying to Me1000, with hashes it would still be trivial to create a system that determines when a friend's signed up, and alerts you. That doesn't require a full address book entry - or any contact details at all beyond an identifier. It's not like Path is actually notifying using any of the contact details it stores, which means it's either representative of wasteful coding on their side, or of something else going on.
The main purpose is to send you a push notification when your friend joins, since a push notification can't execute client side code... that wouldn't work.
