At the risk of stating something obvious, and probably why a lot of organisations teach, and attempt to push people towards rich operations...
There was a historical "myth" that running LoP (lean of peak) would damage engines. This is probably why many POH of old planes often prohibit this. I have put myth in quotes, because, although LoP if done right and accurately, with accurate engine instrumentation has been proved to actually be better for the engine, than running ROP (or over-rich for that matter). Doing it wrong however, could cause both short and long term problems with the engine.
Most spam-cans (pa28/c172/152) that I flew during my training were often 40+ years old, with engine instrumentation somewhat basic and old. Temps being drawn from only one CHT/EGT probe. (whose accuracy I could only guess was roughly working as the needle was moving...) If anyone has ever flown with a digital engine management system with each cylinders displayed, they will know that there is a range of temperatures, especially in old(er) engines. So flying without each cylinders, would be leaving you with 3 unknown temps which may be causing damage without you having the faintest idea.
If you cannot accurately manage the engine temps, you do run the risk of making things worse for yourself. Running on the rich side, with the exception of fouling the plugs (rare above 1000rpm), you don't really have that much risk, and therefore a more cautious option for most of us flying crappy, old and tired planes.
If however, you do have a fancy digital engine management with proper calibration, and installation - you are better (so data tells me) running LoP, but this is often a continual adjustment to ensure proper operations. You can't get complacent about it - saying "oh I did that about 30 minutes ago in the cruise" because any slight difference (OAT, ALT, Pressure) will cause fluctuation and as you are running on the edge, you do need to manage it actively to ensure proper operation. (which is why I tend to prefer ROP operations - and enjoy the scenery a little more)
On a side note, a number of Mooney acclaim and ovations have been reported to have had shortened engine lifespan as a direct result of running over-lean, (so people tell each other) but I can only attribute that to poor engine management (despite their fancy kit).
Back to the main question however, leaning on the ground.... Do you ever remember your instructor telling you to maintain at least 1000rpm whilst taxying, and that you had to close the throttle, and use the brakes to slow down before returning to at least 1000rpm? Well that is so as to not foul the plugs at low rpm. An alternative to this, is to reduce the amount of excess fuel being fed to the engine, hence the leaning on the ground. The main reason why instructors that I know are not so keen on teaching to lean on the ground is because if you are a newly qualified PPL, with 3 friends in the plane, all excited and distracting you, the likelihood of you forgetting something is increased, and they believe (quite rightly in my mind) that forgetting the mixture would not be a great idea on takeoff!
With regards to being on final, I was always taught to go full rich as part of my downwind checks. I don't think it is a good idea to leave it till the go-around, or that moment when you suddenly need full power during final due to severe downdrafts / windshear, because chances are you'll need it full rich, and not have the power you need, when you need it, as a result.