And you can literally feel it, or hear it, if you're blind. Non-native GUIs are very often less than fully usable, or even downright inaccessible for less common toolkits, with a screen reader. The one exception I can think of is XUL-based apps (e.g. Firefox and Thunderbird) on Windows.
Qt's accessibility support is only moderately useful in practice. As one anecdote, TeamTalk (http://bearware.dk/) has a "classic" MFC-based front-end just for blind Windows users.