Expanding things horozontally actually wouldn't be quite as bad (though a few problems might still occur). In fact, I think expanding the jj2 resolution to support widescreen would be perfectly legitimate, so long as the viewable area didn't increase TOO much.

The reason is because most backgrounds rely on being aligned vertically, rather than horozontally. Increasing the resolution would wind up displacing them, creating floating mountain ranges and whatnot. You'd also experience some other visual problems, seeing things you aren't supposed to see (I place all sorts of junk out of the playable area in my levels, such as trigger systems, cutscenes, and so on).

This wouldn't be such a problem with widescreen support. But changing the resolution to something like 1280x1024 would look horrible.