I am a web app user, not a developer and I can see some use cases. Chrome/Firefox/Foo as the main browsers are nowadays packed very heavily already, zillions of tabs, some heavily loaded, usually takes the majority of the system resources. Sometimes, I just want a little browser that can host a few discrete web apps where maybe I just don't want everything crash together at the same time...
I like this a lot. I could see using it for a few certain sites that are accessories, like a music player, or a reference site, without having to drag a tab off and resize.
I'm sorry, I'm a web developer and I don't see the point in this either. Chrome and Firefox have mobile browser simulation features already and I can test my sites in more than one screen size using them.
I am a web developer too, and i can't see any point of this. If i would realy wanna test webapps or responsive websites, i would just do it in a emulator or in my realy phone.
