I would also say that it is bad UX to have them turn it if all it will do is fix a few layout issues, or anything simple like that.
Reasons I think are good:
- Live/Real-time updates (example: stock exchange/rss feeds).
- Performance gain in page load times by bringing in content via AJAX.
- Ease of use, kKeep the user from jumping through too many pages in order to accomplish a task.
- Game/Canvas interaction.