I imagine that the three most common excuses are impatience, being in a hurry, and "who cares -- nobody needs these things anyway."
An MU-2 is a very out in front airplane. Not to be flown as some seat of the pants junkett.
If I were flying over BFE Alaska, I would agree it's a good idea. Flying up and down the Willamette Valley, or even the coast range..it's a complete waist of resources.
2) Radar fallowing? Transiting Class C and B airspace it is good, Flying over boonyville at 2000ft..it doesn't help much. Besides, if you can't see it you are going to hit it, whether they tell you it's there or not. In my experience, people look outside the airplane better when they don't have someone on the ground looking for them.
Both provide a false sense of security.
It's not required when flying under visual flight rules. You might not, especially if you thought you might change plans while airborne. Most people who just go and stooge around their home airport don't bother; but an Mu-2 is not a plane for flying for fun. It's for going places, and as Archangelsk says, it's a very demanding machine to fly. There is a Canadian Mu-2 that comes into Hanscom (KBED) several times a week that is flown with such elan and precision that everybody stops and watches the guy.
A flight plan doesn't really do anything to make the flight safer, and doesn't help you if you are in trouble aloft. It really only helps them find you if you go missing -- they know where to look. Mind you, instrument flights must be under a flight plan (and clearance). All airliners operate under instrument flight rules regardless of the weather. Airliners haven't been allowed to operate visual for almost 50 years.
d.o.l.
Criminal Number 18F