I've got to quibble on this one. The notion of the purpose of the light being to "surprise the enemy" is a bit silly. The purpose of the light is to IDENTIFY the target...which may or may not turn out to be an "enemy". If the ambient light is such that one can identify the target as being an "enemy" (who is posing a deadly threat or who otherwise meets the terms of engagement under which one is operating), then one is better off firing the weapon rather than fiddling around with the light.
As to the issue of position disclosure, it is true that it'll be pretty clear where you are once the light is on. This will be true to some extent whether the light is left on or just flashed. The "school solution" is to use the light in a momentary fashion, but doing so ruins your night vision and you can't see much when you turn out the light. If you confront an attacker, light him up, fire, douse the light, and then move...what then? Now, you can't see him. Is he down? Eventually, you'll have to turn on the light to check out his status. If he's down, fine. If he's not and has moved, you now need to find him all over again.
All in all, it seems that, depending on the situation, you'll either use the light VERY sparingly OR you'll use it for a fairly long period. This and the difficulty of controlling both the weapon and the light switch when opening doors, using a phone, and so forth, make having BOTH a momentary switch and a constant on/off switch the best.
Rosco