I use the designer exclusively because I usually don't need more than functionality that's offered in Qt libraries and because I don't want to spend a lot of time time on the interface.

I think programming GUIs is boring and a huge unproductive time-sink. The Designer saves me an enormous amount of time. Usually I just want to get the main functionality done, and wrapped up in a usable GUI. If I can't build the GUI using standard elements in the designer, I won't take the time to do it.

Besides which ... I haven't taken the time to learn many of all those Qt gui-oriented Qt classes, and if I can use the Designer to get what I want, I'm certainly in no rush to do so.