Funny thing about this topic. I created it, went on vacation, and forgot all about it.
My 2 cents is worth just that, 2 cents. Here it is anyway. I use it all the time. I have NES, SNES, N64, and Genesis on my computer. I use my Xbox 360 controller hooked up to my laptop to play them. I'm a fan. Is it going to keep me from collecting? No. Am I robbing anyone by playing 20-30 year old games? No.
The biggest plus for me is convenience. It is so much easier to fire up my laptop than it is to dig through all my stuff, hook a system up, and play a game. Plus, my couch is too far away from the tv. 6 ft cords don't reach.