I think the idea that 'I have to learn your framework' might be a fairly minor one when you're dealing with html and css. The learning curve is tiny compared to learning Objective-C or Java (for Android apps).
For getting a proof of concept together, and for early stage testing on multiple platforms, you can't beat html. As you mentioned Titanium and it's competitors have some challenges, that honestly, HTML just doesn't have.
That isn't to say that every app should be an html app, just that in early the early stages, if multi-platform, levaraging of web developers (rather than finding app developers) is your goal, a tool like this can be very valuable.
For getting a proof of concept together, and for early stage testing on multiple platforms, you can't beat html. As you mentioned Titanium and it's competitors have some challenges, that honestly, HTML just doesn't have.
That isn't to say that every app should be an html app, just that in early the early stages, if multi-platform, levaraging of web developers (rather than finding app developers) is your goal, a tool like this can be very valuable.