I've been doing a lot of experimenting with weapon mounted light (WML) position and activation. I have a couple different lights. Both have a momentary/on switch on the back. One has an integrated mount and one is in an offset mount.
A lot depends on how/where the light is mounted.
When I get a little extra, I'm going to buy this light:

The tape switch on this allows for both momentary and fixed on.
The great benefit of the tape switch is being able to activate the light without changing your grip. Believe it or not, this is a huge advantage when under stress.