You're right it's not for "big apps" like Photoshop, Visual Studio etc. Yet. A couple of the Build conference sessions indicated this, so this isn't just points of view.
However, in years to come, Metro very well could be used for "big apps" as well. It makes sense to deliver what is needed for small apps first. Then see how things innovate over the next few years with regard to delivering "big apps" through Metro. Everyone is still trying to figure out the best ways to implement touch-first user interfaces. So it makes sense to start small first.
I would not completely write off the concept of "big apps" in Metro yet. Microsoft has huge potential to spring some surprises here. More so than any other player in the tablet market. Apple is fundamentally crippled in this area due to their decision to use iOS on the iPad.
Make no mistake, Metro is not just a "layer" on top of Windows. It is deeply integrated into Windows at all levels. Traditional Explorer is, of course, still there. But it is totally conceivable that there will be different SKUs of Windows 8 that do not actually include the traditional Explorer desktop.
Windows 8 and Metro is Microsoft's formal response to such things as iOS, Android and the rumoured Chrome OS and Amazon Tablet PC. It was a comprehensive and impressive response and it has actually highlighted just how little Microsoft's key rivals know about the software production business.