Aren't you supposed to use water and scrub with a brush instead of rubbing out?
It's a shame they're not exactly like gouache, that'd be great. Lots of people ask about software that mimics traditional media but I'd much rather have traditional media that mimics the Photoshop experience
I realise that no-one's going to invent the sands of time to let us all undo (if only!!), but you know, pen-type drawing, colour, light over dark, maybe even opacity (but perhaps that's a bit too much.) Let's all go away and invent Gouache Pencils! They sound perfect. 
But in the mean time, I still fancy a go with watercolour pencils and buy some I shall. Er... when I have some money, obviously.