Emulators are great fun and ineffably useful.
ROM legality is an issue, and a moral quandary is raised regarding age and availability of the game. If a game is obscenely difficult to get hold of, the original developers and publishers have either closed doors or have seemingly abandoned the software, and emulation of it is victimless then download away and relish the high-quality games of old in all their splendour.
Now for the caveats: if I downloaded a ROM, which for the longest time was not in production, and it then begins being sold again by either the publishers or someone who bought the license, through their new consoles online hub (like Nintendo have done), should I delete the ROM and go out and buy a Wii? I'd say no, I'd have to be nigh on saintly to be that concerned with following the law and making sure someone who now owns it gets my money. If I can buy a game for their new console, and I have yet to download a ROM of it, I'd buy it. It's better to support the industry in all it's facets, even if this latest retro-download-service is a bit really obviously two faced (no effort for lots of income off nostalgia, and they get to look good for "supporting the back catalogue", it's all corporate BS). If I however happened to own the old cart, would I still download a ROM? Probably, since, while illegal, I consider my debt paid to the devs and publishers, since I bought it when money went right back to them.
Morally, it's quite ambiguous, usually these games have made their money and then some, and the new owners (or old owners who just now take notice) aren't being much hindered, if at all. In the case of Nintendo buying games for other retro consoles, and making good emulation for them, I genuinely say bravo and encourage people to go out and buy those. Effort has been made, and they're not very expensive.
Piracy of games that the developers and publishers still depend on to keep themselves in business, is still reprehensible and should be condemned by all gamers in 90% of cases. It's rarely justifiable.