if you own the game, or its no longer 'in print' as it were, its absolutely fine.
other reasons i'd say its acceptable;
the game is otherwise unavailable in your region (i.e the debacle over japenese wii games a little while back, where the developer/publisher has decided they don't want your money)
maybe as a demo, if one is otherwise unavailable, but only for a reasonable trial time (a level, a fight or two, etc),
id maybe accept to experience a version specific feature (unique character, etc) but only if you own a version of the game
also if a remake re-release is available, you may emulate the older version if you buy the new one
other than those you'd need a pretty good reason why you would have to do so, and most of my reasons would be kinda case-by case.
I'll tell you what ever want to know if its ok to emulate something specifically, drop me a line, we'll discuss it over a lovely seafood dinner (your treat)