Well, it might screw up some programs, e.g. speech to text things. I had one (though I can't remember the name) that would let me use a shortcut of whatever the underlined letter was for the button to just say it, for example, if the button was "<u>S</u>ubmit Query" I just said "button S" and it would click the button. I don't know if such programs would be able to get that information out of a tooltip since thats not the standard way of doing it. The feature is useful for some things that have long text like, "I <u>A</u>ccept the license agreement" Instead of saying "radio I accept the license agreement" I just had to say "radio A" and it would check it. Since using a tooltip is not the way it is normally done, I doubt such programs would understand it, at least not without special code to support it.
