Everyone would still need their own legal ROM to use an interpretter. If information on what calculators do with each function isn't in documentation, you need to have a ROM for it.
An interperator wouldn't need or use a ROM at all. It would involve recoding every one of the TI-Basic functions.
Doing an interperator would be interesting (and probably easier than a full on emulator, where accuracy is important), but I'm not sure if it would be helpful or useful...
't would be useful, although some people would complain since there would be no support for omnicalc, xlib, etc