Before there was Qt Creator, I suspect a lot of people were using Qt Designer with other tools. In fact, if you want to develop using PyQt or PySide, I think this is your only option.

The limited integration of this approach probably comes with pros and cons–one pro being that it forces you to learn more of the Qt infrastructure at the beginning rather than when something breaks and your annoyance level is high. On the con side, you need to learn more the Qt infrastructure before you can get going and (more importantly) manually code more of it–which will slow you down and make you less excited about making changes.