In older engines I would tend to agree with you, but in some of these newer ones thicker oil will actually cause damage, especially single-weight oil in extreme cold. Many of the engines produced today have extremely tight bearing tolerances and also have very small oil passages. In a lot of engines the motor oil is used to actuate injectors under pressures exceeding 20,000 psi. The use of heavier motor oil in these engines, especially single-weight oil, can and will cause damage due to oil starvation.
That said, I live in Texas and we regularly see summer temps of 100 F plus. My daily driver is a 1976 Ford pick-up with a 460. I run 20w50 Texaco Havoline in that engine, but it wasn't built to the tighter tolerances these new engines are. What worries me more than the thin oil on new engines is the fact that the oil filter on my riding lawn mower is larger than on most cars and pick-ups now.