>As usual apologies in advance if I am talking rubbish.
Not rubbish at all, but perhaps some might view my reply as such!
>benefit of the sensor would require the use of Nikon's >professional lenses. If this is true does the increase to >24mp make this even more relevant?
Short answer, sort of. Long answer, the dirty little secret they don't tell you.
The D7100 has a pixel density of around 3.9 microns. System resolution is usually expressed in lp/mm (line pairs per millimeter). If I calculated correctly the pixel pitch would work out to about 128 LP/mm. For 35mm format the resolution of the average consumer zoom is about 60 to 80 lp/mm with excellent lenses at about 100 to 120 lp/mm. So yes to take full advantage of the 24MP, you need the best lenses.
But there's another wrinkle, diffraction. To get the most out of the 24MP, you'll need to shoot at fairly wide apertures. If we assume twice the density for resolving detail, then the D7100 would become diffraction limited approximatly above f/6.3 and the D7000 at above f/8.
>then why put large sensors on on cameras lower down the range >which are beginner/improver models?
There is a number of reasons why. One is to improve noise performance. I believe sensor technology used in the D7100 is fairly mature ( there is a new technology in the wings though that might increase sensor performance). So while the higher pixel density of the D7100 probably makes individual pixel performance a bit worse than the D7000, the pixels are smaller and less offensive in the print. Also if you down sample the 24 MP to 16 MP you average out the noise and the D7100 will provide better image quality then the D7000.
At least I think I got that all worked out correctly!