Pots do wear, but you should get a year out of even the cheapies and 10-15 years out of the good'uns. However, with a normal high impedance guitar circuit, the sound
does muddy up as you turn the guitar's volume down - the change in impedance moves the pickups resonant frequency lower.
The way round this is a simple treble bypass circuit on the volume pot:
http://ratcliffe.co.za/articles/volumepot2.shtml