Personally, I think emulation is kinda lame. Playing a rom on the computer just isn't the same as playing and owning the real thing. With an emulator, you're stuck navigating with the stupid keyboard, hunched over in a computer chair, squinting at the monitor. The emulator I *used* to use, wouldn't let me save my progress! Frustration!! When you own the acutal game though, you can check out the cool box art, flip through the manual, etc., etc. Most importantly though, you get to use a familiar controller, and play the game on a nice TV screen.
As far as legality is concerned, I think that if a game is 10-15+ years old, a rom of the game should be acceptable, just because the company that produced the original game isn't making any money off it anyway.
Just my two(2) cents.