Well, the reason I did it that was was because for a field where you're expected to enter letters or numbers (ie for a product key in an installer, for example), all it does it limit your input that way too.  For the plaintext window, really all it disallows are accented letters, and non-punctuation symbols like @ or ~ that shouldn't show up in text anyway.  We could do the validation at the point when we hit preview, but my feeling with that kind of thing has always been "Grr! Then why did you let me enter it in the first place?".  That's my preference, though.  Really it's just tomato tomahto.