They already exist. They do require you to download a .rom file from your real calculator to prove you own the device, however, if you 1) dont feel like downloading the file from your calculator, 2) dont have a link cable, or 3) dont legitamately own a calculator you can find the .rom files online, though such actions are illegal.

Wouldn't this be a little bit redundant if you have a computer to work with anyway? There are dozens of programs that can do that kind of thing plus much more. (Maple for example)

Not exactly. Maple is much too powerful for the casual calc students' use. And paying for Maple, in addition to a portable calculator, is an expense I'd rather avoid.

Also, as far as emulators are concerned, there's the advantage of the calculator interface already being intimately familiar to the user.

This is why i came to this site! My cellphone is many times as powerful as my Ti 86, Why isn't there better software for my windows PDA? Every Ti I have ever picked up is a completely frustrating experience with a new interface. You could have a much faster, more powerful calc with a color screen. GIGs of memory, upgradeable, export to other programs. It doesn't make any sense.

You can get an HP-48 Graphing Calculator emulator from HP Calc