After putting in some thought, I think there could be some workflow benefits.
Firstly, collaboration could be far more streamlined like this. Obviously, you'd need to be online, but that's not a big deal for collaboration to me.
But as far as workflow, it could be awesome. Hear me out. So if it were a possible multi-tab thing, it could really be great. So there are obvious things like layouts and event sheets being in different tabs (if you want). So now drag one of those tabs out into its own window on another monitor. Cool so long as they update each other. Now allow plugins to be in their own tabs or windows. You could work with a timeline plugin or animation plugin in its own tab and watch it update the project on the other monitor. Cool, now have a teammate doing that on their own machine while you do control work. You could multitask yourself or with teammates across operating systems in near-realtime (probably 'save' could be an auto-cloud-push). Then maybe plugins are paired to your account but hosted by Scirra. Have the stuff available everywhere even if your work rig is not.
And that could all be possible without any teammate having to install anything. Ever. That's a service and workflow I can get behind.
Now is this how C3 will actually work? No idea! But I think there's a ton of potential to this and I'm interested to see what Scirra has come up with.