I think that as far as steam cabs go you can get away without the 'correct' cab as you spend most of your time leaning out and watching the road, unless your firing! So as far as the 40 and other non steam locos are concerned a cab is essential.
There also seems to be mixed views as to wether RS is a simulator or a virtual model railway, hense the argument for cabs etc, well to me it's both so we need cabs, but that is up to those who make em!
Sound to me is very important, we love the 40's not just for their appearance! The default steam sound is very good but if you add a big end clank to the black5 and the sound of the tender bogies the atmosphere is 10 fold. So just imagine what the 40 would be like with proper sound, the 25 is great fun, once you put Nikos's sound on it. Taking that as an example, I use it in scenarios as AI and I'll use it on a train I'm driving, but I prefer to ride behind it using a 'head out of the window view' rather than sitting in the alaised 47 cab.
Mark