Tassilo, do you know what is the reason given by users to like the less
powerful built-in modes? Presumably they don't need preview-LaTeX, but
what else?
There's also some psychological comfort to knowing all the entry points to the software one uses, which is easier to do for tex-mode than for AUCTeX. When all else is equal, I generally prefer simpler software (or packages) that I understand completely over complicated software that I have trouble wrapping my mind around. I think this is essentially the "feature bloat" criticism noted elsewhere in this thread.