I understand that making emulators is tricky business in all aspects being it the mere creation of it, or the legal implements of it. I'm not quite sure how the legal thing stands with it (well Nintendo have patented emulation of handhelds (or was it emulation on handhelds) now, so if that holds the law would be clear there).
I can clearly understand why someone who put a lot of work into an emulator would like some sort of compensation. It's about ideology. Some people can afford to do the work for free because they care about the thing a lot, have a financial situation which allows them to continue with it, and would rather see their software used and have made a lot of people happy.
Others are in it purely for the money. Then there are the people inbetween. Maybe they really need the money and won't take a chance on donations. Maybe they feel that they've put a lot of work into it and that they should be compensated, not just by donations. Whatever the reason I respect that but they shouldn't start bitching if people aren't using their emulator but an alternative. Once you start charging you are entering into a competition and you'd better be certain your product is well enough to compete.
A lot of the programmers working with Linux/open source software in general do the work because they really want to, not because their economy depends on it.
For systems like the PS2 and PSX and Xbox emulation could be an actual plus to them. Some people do go out and actually buy the games instead of copying them. I think most of us know that Microsoft isn't exactly making money off their system, but from the games. Same goes for many other consoles, it's the games they make money from. If there were to be a way for people to get access to play the games for their system without having to shell out the money for the system itself it'd only be a plus since that'd gain them a customer they wouldn't have had if the hardware had been nescesary.
Companies like Nintendo who use a propriatary format to store the games on would be much harder pressed with emulation. To play the emulated games there's no way but copying the games. You just can't go buy one of their old carts and plug it into your computer and have it play it, neither will their GC discs work with a PC CD-rom drive (as far as I know).
Again back to paying for emulators. If the product is good enough I wouldn't have a problem paying a small fee for the emulator if I was going to use it anyway. If I were playing on my Amigas more I'd buy WHDLoad... I know that's not exactly an emulator, but I like to think of it close enough. It allows you to play older games with the convenience of them working on your new system, faster and at times with more features. There are old games which do not work on a stock A1200 unless you use WHDLoad. For upgraded amigas this problem only gets bigger. WHDLoad solves this problem. The most different thing about WHDLoad in comparison to most other emulators (from a copyright holder's point of view) is that most WHD installs check that you're in posession of the original disks, something which you just can't get an emulator of the old NES system to do.
Emulation is mostly connected with pirate software, and as such it might not be a good idea to openly make money on the emulator since this will make them a target of a copyright holding company. If you look at emulators for the Amiga and the PSX you'll notice that they require you to have the rom from the system to actually use the emulator. This is a good security measure to take to reduce the risk of getting hit by a lawsuit.