I've played a couple of night games of paintball with a mounted light, and it was a huge advantage. Reloads and manipulating any other gear became a real hassle with a non-mounted light.
I've also used lasers on pistols and carbines, and if I had my druthers I'd probably prefer a light on a long gun. I will admit to having a fun time with a 10-22 equipped with an early laser that made hits at 100 yards from the hip a breeze, but then again I didn't have to ID which soda can I was shooting at.
Where you mount the light makes a difference. I found underslung lights leave huge shadows behind low furniture. Side or top seemed better.
Yes, you can impact your night vision with the light 'splash' from light colored walls (I can attest that a tree trunk at 3:00 AM will do it too). Using the light as little as possible seemed to be the way to go.
If you illuminate the target with your light you can see your sights- black, night sights, aperture, etc.