Locking in users and their data removes incentive for developers to please their users.  If you know that all your user could cancel their service contract and be set up with a competitor within an hour without losing anything, you have a damn fine incentive to keep those users happy.  If on the other hand moving to a competitor means losing access to years of data and weeks or month of work just to get back to where they started then you have to suck pretty damn hard for people to leave you.