I use mspaint all the time. I mean I am a frackin' mspaint wizard. I have the Gimp installed on my system, but 85% of the time I use mspaint anyway. I know that the Gimp is superior, but I've used paint for so long and have gotten so fast at using it that I can usually do whatever I need to do with it faster than with the Gimp just because I'm much more familiar with mspaint. Even knowing the Gimp is better and it would probably only take me an hour or two to get pretty good with it I still choose the lazy way most of the time and fire up paint.
Interesting (though pretty obvious) explanation for product loyalty and all that-- but what about people who use more than one product.? I'm pretty sure that at least some of people's brand opinions are based on objective reasoning and not always familiarity.