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.
What tabs wouldn't provide you is the separation of resource management. Your little tabs would die, refresh, reload with the rest of your main browser (there may have other limitations, e.g. cookie size limit). Is that a good thing? Not always, at all time, at least.