From what I have observed, many people do not, which would explain how they talk about their legs killing them. I don't like driving in heavy traffic, but I don't like it any more in an automatic car.
It might depend on your size? I am tall and long-legged, so the effort of using the clutch is lifting my leg above the pedal and feathering it. Holding the clutch to the floor is no different than resting my foot on the dead pedal next to it. The only reason I would not want to hold the clutch for a long time is if I am worried I would be distracted and forget I was in gear.
I would never put my manual in neutral unless I am about about to set the parking brake. It represents a mode shift in my attention and vigilance. I always did this startup sequence (and its reverse for shutdown): foot on clutch, foot on brake, start engine, release parking brake, transmission into gear, foot off brake and over gas pedal, feather clutch to pull away.
Like many on this long thread, I eventually compromised and got an automatic because of the narrowing options in the new car market here in the US as well as to make my wife comfortable sharing the car.
I am learning to enjoy the extra gears and the aggressive automatic shifting program that gives a quiet, comfortable cruise. With a manual, I would never shift my way so close to the idle speed when cruising, because my ear would be telling me it is time to downshift. But, knowing that the computer is handling it means I enjoy the comfort and can be impressed by the fuel efficiency in this mode.
I do miss the engine braking though. I think automatics are tuned to an irritating preference for coasting. I'd much rather have to keep the gas pedal depressed in order for it to sustain speed, and to have it automatically downshift and slow the car when I start to raise my foot. I'd like the automatic to do this all the way from highway speed down to the slowest crawling speed in the lowest gear, without any other inputs.
Newer electric cars (with regenerative braking) have a "one pedal" mode like you're describing (it engages some amount of regenerative braking by default, unless you have the accelerator depressed a little, so you can just take your foot off to slow down).
