Triaging issues, sure. Progressive enhancement? If you’re parsing User-Agent to implement that, you’re doing it wrong. (Feature detection is the correct approach, when necessary.)
Edit: OK, this isn't progressive enhancement. But it's still a major problem you have to sniff the User-Agent for. I don't want to sniff UAs either but when Chrome is wont to change how fundamental parts of the web work like cookies it's sometimes necessary. I know UA hints will still allow you to do this, but requiring a second request is going to make things like redirect pages difficult to implement.