Trying to decide if I should spend some money this christmas and buy myself a light that I could either put on my Ar-15 or my G21. Are they practical or are they a good way to get yourself shot? Do LE use them in real scenarios or are they mostly just a movie and video game gimmick? Some solid advice would be appreciated.
People like them or they don't in regards to perceived tactical benefits. Yes, LEOs really use them. Yes, lots of homeowners really use them. Be it a weapon mounted light or a hand held light, it is important that when using it that you use it effectively. Both methods of deployment of portable lighting have advantages and disadvantages and depending on your perspecive and perceived needs, one way or the other may sway your opinion. That it is on the weapon does not make it offensive and the need for one isn't an offensive or defensive issue per se, but an illumination and transportation issue.
As for the balance issue noted, the gun's 'balance' changes with every shot fired. Balance is not a serious negative issue with a weapon mounted light as implied. If the balance is a problem for you, then you probably can't handle the recoil or the balance change as the gun is fired. Think about it. The weight of a fully loaded .45 acp 230 gr. round is .74 oz. So the gun changes its weight by that much with every shot and so balance changes as a result with every shot. Considering the gun only weighs about 38 oz fully loaded, a change of 9.62 oz would be a significant balance change when fired to empty (weights are for OP's Glock 21).
However, that the balance does change by adding a light, is true, and for most people this is actually a benefit. The light adds 1.5-3.5 ozs of muzzle weight, thereby reducing muzzle rise during recoil.
Wires and pressure switches? Maybe somebody sells handgun lights with pressure switches and wires still, but I haven't seen them for sale in years. That is pretty out of date and old school technology.
Holsters? Not really a problem at all for common guns like the Glock. Additionally, most WMLs that you run on a pistol attach and detach very quickly. You can run it as a handheld light or as a WML, and option not available to simple handheld lights, especially not those from the dime store.
One real benefit of a WML is that you don't have to leave flashlights all over your house to have access to it, LOL. It rides with you on the gun if you carry it in a holster.
Also, a WML does not negate the use of another handheld light at the same time. You can have both.