It's unnoticeable and hardly takes up any disk space or system resources.
Rosetta is more than the simple file you download. It requires them to have system files compiled for two architectures. Moving to intel-only will make things simpler and easier to maintain.
If I can play Nintendo and Commodore games on OS X with an emulator, I'm sure there's a way to run Power PC apps with an emulator. If it's not Rosetta, someone else will write a program to do the same thing.
They might, but it will suck, be a pain to set up, and not work with the majority of apps. (Like Sheepshaver.)
BTW, I've heard people say that you could run Snow Leopard as a VM in Parallels, is this even possible? I thought you couldn't install OS X in a VM.