I like the adjustable difficulty, though I do try to stick with what I started with at the beginning of the game. But take Modern Warfare 2, specifically: The Favela level where you're running through the streets of a town filled with 3 story buildings and every window/door/roof has at least 4 pissed off south americans shooting at your face. I believe the mission is officially called "The Hornets' Nest" and it's quite aptly named because you're literally getting shot at from all directions. Call me a pussy, but I couldn't get through that level even on frickin' Normal difficulty, I had to bump it down to Recruit which was particularly embarrassing because I had gotten up to this point while playing on Hardened!
So yeah, without adjustable difficulty I wouldn't have gotten past that level and would have had to start the game over. So really I don't see a problem with it.