Canon Lens question?

[NateWagner]

well, as you said USM is quieter which though you might not hear it from 15 feet away, in a church at a wedding where there is utter silence it could still stand out.

The MAIN purpose of the USM though is that it (the USM equipped lens) tends to be much faster at focusing. This can be a highly valuable asset to many.

In addition the ability to have manual over-ride and the front focus element not rotating are both beneficial in that the latter allows for circular polarizers, etc. and the former allows for micro adjustments particularly if a lens is having a difficult time focusing and you need to take over or just modify the focus a tad.