I own two two-strokes with oil injection pumps, and have owned several more in recent years. I would never disable them. I have only ever had problems with one when the pin on a Yamaha pump came out and the pump wouldn't deliver at more than tickover rate. That did make it sieze, but not terminally and I did manage to limp home.
If anyone does decide to use premix there can be problems with certain brands. Yamaha's are easy enough to do as the oil is injected into the inlet tract, so it gets around the engine in the same way. With the Suzuki's I have owned at least one main bearing is lubricated direct from the pump, with the oil finding its way to the rest of the engine via a 'flinger' plate on the crank. This, presumably, would have to be removed or oil would not be able to get to the bearing. I can't speak of Honda's, Kawasaki's or Bridgestone's, as I have never had the pleasure of dismantling them.
You are right about the oil on the rider's back. Not too long ago, when I had a high-pipe T250, I bought a new jacket. Unfortunately it was a silver color, but not when I returned from the ride! Even on my TC90 I would come home smelling of burnt two-stroke. You are right too, when you mention the poor quality of oils 40 or 50 years ago. Decokeing the muffler of my Suzuki 80 was an all too regular event.