Two of our aircraft will foul on the ground if not leaned out. I only run full rich for run ups and as i'm lining up. I was always taught to guard the throttle quadrant with my hand across all 3 (2?) in the take off roll, an out of place mixture would soon be found. At worst a little bump up to full rich as the throttle goes up and a little mental kick in the ar$e.
It's not a good look for your pax if you haven't leaned it out on the ground and during the mag check she starts coughing and shaking.
Also with our Turbo Lance (with a wonderful little digital fuel flow meter) the full rich fuel flow in cruise is around 90-95l/hr!!! Leaned to max power you're down to 60l/hr. Off the top of my head (with a smidge of help from the calculator) that's a range (with reserves) of about 780nm as opposed to 525nm. That could give you a nasty little fright 250nm from home!!!
It's not a hard thing to learn to do properly, there's alot more complicated stuff in that cockpit that's for sure! If they're not teaching you how to use that little red lever properly, what else aren't they teaching you?
A bit off topic, but ContactMeNow's post reminded me. Years ago i was shown an easy way to get a rough fuel flow for a properly leaned engine running at about 65% power:
Double the horsepower and take off a '0'. Simple
I'm only talking Conti's and Lyc's and none too complicated, but has proved to be a useful little formula over the years.
182 (230hp) eg. 2 X 230 = 460 = 46l/hr
PA-32 (300hp) = 60l/hr
172 (160hp) = 32l/hr
PA-44 (360hp total) =72l/hr
Again, only as a rough guide, but it's been proven remarkably accurate time and time again.