Yep, I agree. Which is why I said depends on the aircraft and situation. As when I'm towing if I shut down for a minute, start up is on one mag, then the other, so you do actually do a check then, as you can tell if one or the other is actually working. (it sounds pretty dodge on just one!)
Sometimes after a quick turn around I'll just flick to each mag briefly to see what things are like (but no runup)
It all depends on the situation, what you're flying, and where you are also, in my opinion. I always cringe when I see pilots at gravel/dirt strips do high rev run ups stationary with dust and stones flying everywhere. Not to mention carby heat!
Just on another note, what are peoples opinion of doing run ups wearing sound deadening headsets? As a mechanic I can tell when something is not right with the engine, but you need to hear it to know.