I'm surprised so many people are seeing this as one or the other - that the choice is limited to nodes or layers and that's it.
The fundamental operations of image compositing are all pretty much completely the same. On a mathematical level any application that can composite images together works exactly the same. The most commonly used compositing operation can be expressed simply as...
Composite = FGalpha * FGcolour + (1 - FGalpha) * BGcolour
All your presented with is a different way of manipulating those components (FG, BG, alpha) in the UI. It's entirely possible to have a node-based system operating in the background and just present the user with a layers UI.
But I don't think Nuke is suited to raster painting - yet I do see lot's of potential for Mari to be developed further in this area. I'd love to see a 'canvas' tab added to Mari, where instead of storing the paintbuffer in memory (like it currently does), it would store the paintbuffer directly to disk.
While Mari doesn't have all the fancy tools Photoshop does and I'd like to see some more features (dynamics) added to it's brush system - I very rarely have to go back into Photoshop these days to do any textures work, about the only thing I use Photoshop for is creating text elements.
I'm really interested to see what's in store in the upcoming Mari 2.0 announcement.
And with a little imagination it's not hard to see Photoshop with instanced layers - would make it infinitely more useful.