Shannow, if the bike has been jetted up correctly for use with a 50:1 fuel/oil ratio, going to a 20:1 ratio could leave the bike dangerously close to piston meltdown, as lean conditions run extremely hot. I have seen this on many occasions first hand.
Try holding a lit match to a cup full of Yamalube. Now try the gasoline....
Besides, very little oil reaches the combustion chamber, IF the engine is metering the fuel mix properly.
Some bikes do carry a very rich mixture recommendation, and I suspect it to be a CYA thing, as Panzerman describes it. However, the "jet is upped on size" thing is not really the case.
The mfg knows that the correct ratio is dependent on engine and use specifics, and that proper metering of the air/fuel/oil mixture is dependent on many variables such as quality of fuel, air temperature, relative humidity, and elevation.
The mfg simply cannot provide a catch-all metering scenario, as his equipment is sold to areas with a wide array of conditions.
The jetting chosen for a factory bike is basically an arbitrary median average. It may be rich for one man, but the next man may find it lean.
Many will want to add less oil to their 2cycle engine when they see a dark, oily spooge coming from the tailpipe. It would seem logical that if there is oil coming from the tailpipe, that the mixture is carrying too much oil...
But that is again where conventional wisdom separates from facts.
When an engine is producing this oily residue, it is probably due to the mixture not being metered properly by the carb, and as a result much of the FUEL and it's components gets past complete combustion, and turns itself into an oily looking substance along with a small amount of the oil that didn't fall out of suspension prior to combustion.
Adding less oil will not fix the spooging problem, but rather enhance it, since you have now added more fuel to the mix...and the metering devices (jets, needle, etc) are already allowing too much fuel into the chamber to begin with. The result is even more oily fuel spooge, and the engine a bit less powerful.
A properly set up engine, that is metering the fuel/oil/air mixture to optimum, will see most every bit of the oil fall out of suspension prior to the combustion chamber. If the remaining fuel/air mixture results in about a 12-16:1 air/fuel ratio, then the resulting burn will be clean and powerful.
The goal of the tuner is to find this scenario and keep it there. It's a moving target, so the pro tuner has to be very sharp about these things, and know exactly what effects the changes make.
As there is no catch-all weather, there is no catch-all jetting.
But the optimum jetting can be found at any given time, and the fuel/oil ratio is yet another tool that can be used to find the optimum burn.