There are only two situations I know of when downloading a ROM off of the internet could be acceptable.
1. You own a physical copy of the game, but the anti-piracy protection prevents you from copying it yourself.
1. it's not legal. even if you have the game, unless you backed it up yourself, it's still illegal. if you bought a copy of pokemon and decided to pirate a bunch more, nintendo wouldn't lose any money, but it's still stealing games. (even if you wouldn't have bought those extra copies).
it's not legal, but it's not stealing either: it's going against one law to be able to enjoy the rights you have from another law (a personal backup copy).
2. The game has neither been released in your country nor in a language you know, so a ROM with a translation patch is the only way you could play it. (i.e. Earthbound Zero"]
2. not true. the only way this would be legal is, again, if you backed up your own copy and applied a translation patch to it. for your information, earthbound zero is not a translation, it is a real game that was just never released. somehow it was leaked, so it's not an official game, nintendo cannot make any money from it, it is perfectly legal to "pirate" this game.
i believe that if the game is no longer available or the company that released it can no longer make any money from the game itself, it is not illegal to "pirate" the game. such companies are trying to prevent piracy this way by re-releasing old games through things like xbox live arcade and virtual console. however, nintendo still refuses to release earthbound on virtual console, so i "pirated" it to spite them, since it's not "illegal".[/quote]No, it's never legal do download a game, even if it is no longer sold anywhere; the terms for public domain depend only on time passed from an event (publication or death of the author), and they are so long that no computer game, not even tennis for 2, is even close to reaching it.
Even if the game was never published, copyright applies at creation, so you wouldn't be allowed to legally download / share it either; additionally, you may be violating some other law on industry secrets.
While being illegal, however, "abandonware" (as opposed to unreleased leaks) does no damage to anybody, and thus there are little moral grounds to keep it illegal.