It does seem to help a little on some types that are a bit prone to plug fouling during extensive taxiing, (E.g. 182RG.) The trick is to remember to put it back to fully rich before take-off. I knew a guy who lined up, opened the throttle fully for take-off and the engine promptly stopped. (Slightly embarassing!)
As for leaning in the air, its often essential if you want to actually get to where you are supposed to be going because the book fuel burn figures are always based on a properly leaned mixture. Also, to get max power available, lean for take-offs from hot and high fields. Personally, I've never come accross any but if some schools are teaching not to use the mixture control, they are a bit remiss. An EGT gauge is a great idea to help get closer to max power, max economy etc. Just shows you how 'old tech' aircraft engines are - a mixture control? in this day and age? The motoring fraternilty worked out how to get rid of those back in about 1920! Role on FADEC.