Larry Vicker's is a huge proponent of smaller/lighter = better on an M4 carbine.
He long been advocating the use of an X-300 light on the rail foreend in front of the front sight for a long time; a micro aimpoint; leaving the quick detatch buckles off of the sling.
He also says that anything much over 100 lumens for close-up work is detrimental (lose to much night vision from the bounce).
Anytime you make something smaller and lighter on the rifle you have a win. Lighter when carried, and less stuff sticking out to get hung up on kit and door frames is bueno.
A few others from Vicker's unit have said the same thing, so I think it is an organizational preference for small and light, forward mounted with no tapeswitches. This is different from SWAT types who seem to prefer big and powerful with tapeswitches and/or vertical foregrip switches at the mid point of the handguard.
The thing to remember is soldiers (even those in the counterterror realm) and police officers have very different jobs and what works for one may not be ideal for the other.
I've experimented with my X-300 railed to the front sight post, but needed a laser and decided on a Viridian X5L. I got some training with this setup (under the instruction of a guy from Vicker's former unit) and it works reasonably well. A tapeswitch, for my purposes, would be nice, though.