"Why make it complicated?"
Because I was not happy with any of the oils that I tried- poor shifting, poor clutch feel, or too expensive.
I think I get the best of both worlds with my semi-synthetic mix, and if oil mixing was a problem, every time someone switched to a new oil, the remaining old oil in their engine would cause a problem.
I have used a mix the last 10 years, and I plan on using a mix for the next 40 years (I will be 96 then). I like that I can tell if the oil is just starting to fail by how the transmission shifts, for my motorcycles.
For my cars, I do not mix oil. I buy Valvoline MaxLife (a semi-synthetic), and my wife takes hers to the dealer for the recommended synthetic oil every 5,000 miles (for a 2013 Mazda still under warranty).
“Banking institutions have banished precious metals and substituted a more fluctuating and unsafe medium; they have withdrawn capital from useful improvements and employments to nourish idleness for the few, who prefer demoralizing pursuits to labors useful to the whole; the peace of the whole is endangered by evils more easily to be deplored than remedied." by Thomas Jefferson,1810 (edited)