I think your custom titlebar helps set the font you like. But some other problems will appear:

1. The configuration (position, color etc) of control buttons (minimize, close, maximise) will be different from button configuration of managed (by window manager) widgets. Also the color of titlebar will be different. It will not be good for some sort of applications.

2. Implementing your titlebar you notify window manager not to handle your widget (Qt::X11BypassWindowManagerHint). It means that you should write code for manading your widget (moving, raising, iconifying). Will your widget obtain the focus by the same way as other windows in the system? I think, no.

3. You should test whether your widget will appear in the window list? I'm not sure. I think, no!

4. May be, some users dislike your font.

XMMS has its own titlebar but you can see the widget name in the widget list. I think, that you shouldn't write your custom titlebar without absolute necessity.