The cat(egory) refers to the data speed it can handle.
Cat 3 is overkill for phone at 16mz but if you have 2 lines you still need twisted pair so that is about as cheap as you can go. Cat 5 and cat 5e are for high speed data but that is becoming very common in houses so it is the defacto wire people install for everything. With the right terminations you can carry video, audio or just about any other kind of "data". Unless you do enough low voltage to carry a couple different spools of wire I think I agree, just use cat 5e for all your phone and data wiring.
I still don't think it is right for doorbells, garage door openers and thermostat wire (too small) but I see it.
