I wonder if the derailleur really needs replacing--"you need to replace it!" seems to be a standard phrase at many of the bike shops.

A FD is pretty simple, spring-loaded with a couple of pivot points, and it seems odd to be sticking. I would start by a) servicing or replacing the shift cable and b) cleaning and lubricating the FD. Are you sure it's the FD itself that's sticking instead of the cable?