Andy- well, yes and no. I'll give you that on stuff like Slick-50, Marvel's Mystery Oil, and STP Oil Treatment. But you gotta admit that a reputable brand of $2.00 carb/injector cleaner every couple thousand miles certainly can't hurt, and they are generally regarded as a good thing to do. At least for cars anyway.
I never heard any recommendations one way or the other about bikes. Maybe if you live above 8K RPM you don't have to worry about it. So I should be fine.
I agree about the aint broke-don't fix thing, but how about the spoonful of prevention is worth a pound of cure thing?
I have never used an oil additive - IMHO don't use them if for no other reason than the wet clutch may not like the additive. Just use a good quality motorcycle oil and change it often.
Fuel additive - I have had a fuel tank rust through a few years back so every month or so I use a carb cleaner additive that absorbs any condensation that may accumulate in the tank. Maybe it helps maybe it doesn't but either way it's only a few cents in the long run. Another thing I try to do is make sure the tank is full when I'm not using the bike for a while as this also helps to reduce the condensation problem inside the tank.
Fuel : I just use premium unleaded which is supposed to have solvents in it. Occassionally I will 50/50 mix a tank of premium unleaded and leaded ( or lead replacement ) fuel, just to give the valve seats a break. Don't tell the EPA.
Oil : I only use top quality Shell Advance and change it regularly. Normally I wouldn't use synthetic in an air cooled motor but it seems to work fine.
The only time I use STP/Motor-up etc is if I have an oil burner I am trading in to a dealer. Need that top $$$