The vast majority of the pictures I take are run-n-gun candid shots. I just like this style for some reason, both indoors and outdoors. I use M-mode indoors with flash, but even in that situation, the camera is still heavily influencing the result via ETTL flash metering.
Outdoors, there are so many variablesshade vs. no shade, angle of sun as everybody (including me) moves around, clouds zipping by in the skythat I just dont see any benefit at all to using M mode for the types of photos I like to take, unless there is some oddball lighting situation that would confuse the camera too much.
I go to Av mode, set the aperture for the desired DoF effect, set the ISO to as low as I can get away with, and monitor the shutter speeds in my viewfinder as Im shooting to make sure they stay fast enough for sharp shots. For good measure, I chimp the odd histogram. Easy.
I realize you can get pretty quick with the manual controls with practice, and in fact I am reasonably quick with it considering I only have one dial, but when youre watching people through your viewfinder trying to identify those priceless split-second candid moments to capture, you just cant be quick enough.
Besides, aside from certain special situations where I either want blurring or the subjects are moving erratically and I want to freeze them (sports), shutter speed usually isnt important from a creative standpoint, so why bother fiddling with it manually? In most situations (for me), its only important insofar as I need to know its acceptably fast to defeat camera shake. In that sense, its monitored in a binary pass/fail fashion rather than something that is dialed in to suit my creative intent for a given shot.