Why? I don't own a gameboy and never will, therefore I am never going to buy a gameboy game. If I was to go and pirate a load of gameboy games for the purpose of emulation nobody loses out, because there was no chance of me buying a game I'm unable to play in the first place. On the other hand, I have a PS2 and still buy games for it from time to time. If I pirate a PS2 game the developer has lost a sale, this doesn't apply for gameboys.
You could argue that Nintendo lost a sale of a Gameboy, but the way copyright law works I think older console emulators technically get around that. Nonetheless, you're still acquiring a copy of their work for nothing. They're not really losing out on a sale (unless they started selling ROM images, which would be interesting), but the proper way to go about using the emulator would be to pay the company for an electronic copy of the game that ran in the emulator (fat chance, I know) or to buy the cartridge and use whatever hardware is used to get the ROM image off of it to play in the emulator (also fat chance, I know).
Gameboys are a bit different from a more modern example because you generally can't find the games for sale anywhere even if you tried to find them, or the consoles themselves for that matter. My morals are a bit flexible there, especially since any revenue generated isn't going to go to the original author of the game now anyway. Any copies would be second hand.
For a
slightly more modern example, consider the Playstation. Emulators existed that could play the games if they were put in a PC's CD drive. Would you still pirate the game if you had a PC emulator, or would you go buy the game and just skip the Playstation part of the equation? I'm not sure, but suspect that the modern game consoles can be used similarly with emulators, although they probably require illegal firmware copying unless you own the console (the Playstation did).
Why? I wouldn't buy an album without hearing a few tracks first, a book without scanning through a couple of pages, a car without taking it on a test drive or a sofa without sitting on it. I'd actually say it's kinda foolish to do otherwise.
That's what the demo is for. Most games have demos. If they don't have a demo you can actually download, such as for console games, you can still often get plenty enough information to decide if it's worth risking the money on it. PCs have the added level of complexity in that the game flat may not work, but again, that's what the demo is for. If the company producing the game doesn't release the demo, that's a risk they're taking by not letting the fans try it out, although it bites the fans too.
The example about music is simlar. You can usually find samples of music tracks on iTunes or in the store.
To be honest though, this one I wouldn't
really care much about if people are honest about buying the game if it works and they like it.
1. I own the the disc for X console game. I want to play it on the CPU. Justified enough for me.
I'm flexible about this too, but so far haven't been bothered enough to actually pirate a game based on this.