Aren't the bifocals just a convenience over quality? It's inconvenient to wear two sets of glasses, but why would one wear bifocals while driving? This convenience can come at a high price!

Bifocals in general are quite useful. It's nice to be able to see the road and the speedometer using the same lenses.

Traditional bifocals and progressives are different beasts. The hard outline on traditional bifocals means you get essentially two different lenses, both able to function as intended. The soft blend on progressives means you get essentially one big blurry lens that does not have well defined properties anywhere.