Quoted:
Having gone through multiple no/low light classes and many classes otherwise involving no/low light shooting plus living in the sticks where we shoot and practice at night, I say momentary only is the way to go. Mr. Murphy says that I don't have half the control I think I do and the wrong shit will happen at the wrong time unless it's "dumbed down" to purposely on or otherwise off.
I agree with this, assuming you're going to be in a running gunfight.
However, I think that for the majority of scenarios, people are going to be better served with a light that has the ability to perform both momentary and constant on, depending on necessity of the application.
For example, if you are in a gun fight, you want the ability to "blip" or "strobe", i.e. using the light in short bursts. However, once you've downed or cornered your target, you want to be able to keep the light on them, and possibly have a free hand to operate a radio or cell phone, without having your light turn off because your support hand is no longer on the weapon.
I'd highly recommend a Surefire 6P with a Malkoff M61, and either a Z41 tailcap (where you can rotate the tailcap if you need constant on), or even better yet add a XM07 tailcap which has both click switch and a place to add a pressure pad. A Surefire Scout light is also an excellent alternative, which has a click switch, and you can either use it in stock form which throws like crazy, or add a F04 diffuser for a more dispersed beam, incredibly useful at close range.
JMO, and as always YMMV.