I do understand your frustration but I don't think your being entirely fair. I think they could fix the 16 shader limit if they wanted, its just not seen as a major issue. After all Terragen is supposed to be used in conjunction with other apps. If you want the great shader support you can get through Max, Maya, XSI, Lightwave etc then use those packages and learn some compositing skills. I would personally be more interested in better overall shader integration, as in proper transparency and the ability to mask the specular channel for example. There is an issue with Poser exported models in that they come out with an inordinate number of texture channels, often mostly pointing to the same image map. This, although annoying, can be easily fixed and simplified (do I really need a separate channel for the tongue and the inside of the mouth that cant be seen on a model that fills only a few hundred pixels of an image?)
Texture layers would be usefull (using one channel with several UV maps)
Masking of the specular channel (and separately any other missed channels)
Grey scale transparency
Light and/or camera incidence angle (think it can be done through functions ...)
Just my groats worth
Richard