Ok, at the _extremes_ of both rich and lean:
Given that it isn't rich/lean enough to preclude running:
Lean: the bike will be running extremely hot during the operation of the lean circuit, be that choke, idle or mains (or a combo). This can quickly lead to death of the oil, clogging of the filter with carbonized bits large enough to be trapped. Hot spotting can take place within the water jacket where portions can overheat and cause microboils. Eventually, metal on metal contact will take place due to the fast breakdown of the oil barrier, resulting in scuffing and eventually seizing and such wonderful crap. Somewhere along the line, backfiring can occur, since periods of too-lean for combustion will occur, resulting in vapour buildup in the exhaust headers. As a 'good' combustion stroke occurs, the expansion in the cylinder head will ignite the mixture in the header, causing the 'boom bang'.
Rich: the intakes, plugs, valves and pistons etc will continually be getting washed with one of the best solvents around, ie gasoline. This will remove the 'dry' additives left on the cylinder walls after the engine is shut down, removing very quickly the scuff resistance of the motor during startup. The carbonized bits will develop a life of their own as a sort of glow plug, acting as preignition points, further screwing up the 4 cycle process. After a time, plugs will be firing more and more intermittently, allowing more and more gasoline to wash down the cylinders and contribute to acceleerated oil breakdown. If you are lucky, the engine will cease running BEFORE the oil is such sludge that it replicates engine failure as in 'lean' above.
Going strictly by the spark plugs on a 4 stroke STREET bike is problematic at best. If the pilot circuit is lean, but the mains aren't bad, the plugs may not reflect a lean condition. Vice versa is true, ie mains lean and pilot good/rich etc. Back to my recommendation of dyno tuning, especially with a sniffer to see what is going on within the engine (as reflected by the back wheel vis a vis throttle settings) and coming out the chute, via the sniffer. Anything else is just guesswork, sometimes very educated, most often, just a WAG (Wild Ass Guess).
:rider