Hi,
I don't think MELABS is going to add support for new devices in the old version of the compiler but if you're only using devices supported by your current version and you then that's fine.
There are no new BASIC commands added in PBP3 but the REAL advantage (for me) is the conditional compilations feature. On top of that it adds a big improvement to the way the CONFIG bits are handled. No more editing include files etc.

So it's really up to you but for me it wasn't hard to decide.