.. Or you could be running or moving explosively while reloading, making a glance at your magwell an opportunity to stumble.
Think of it like an ideal shooting stance-- its a luxury that you take if its afforded to you.
Disagree here. I do a bit more running around while reloading than most. If you can chew gum and walk at the same time, you can still look the mag in while running, and even do so while running to a very specific spot. Even in less than ideal ground conditions. (the mud is ankle deep in a lot of spots in the video below, and there are pieces of 2x2 fault line hiding in the mud threatening to trip)
Why? because your eyes are only on the magwell for 0.5 seconds +/- .1, and do have peripheral vision that works just fine for that .5 seconds.
Running around trying to reload is actually that much more reason to look it in. When you're running or moving, that perfect standing reload "muscle memory pattern" doesn't really apply any more. The gun is bouncing around a little, you're moving around a little.
[video=youtube;o93ccabWRMk]http://www.youtube.com/watch?v=o93ccabWRMk[/video]