Well, in E6, I can use 1st level fighters as opponents against the PCs for the whole game. They're going to go down fast, but they're still credible threats against 6th+20 feats.
But 8th+20 feats? Not so much. That's when you're looking at level 3 or 4 fighters; they go down fast, but I find tracking their hit points and status effects is a lot more cumbersome than level 1 characters. Also there's the element of world design; if level 3 and 4 fighters / thugs / whatever are so common that they are the "basic mook" of the campaign, where did this supply of level 3 and 4 fighters come from? How did they get to level 3 while mostly spending their time patrolling?