I don't neccessarily think it's such a horrendous idea for Rob Zombie to host TCM Underground AND I don't think it's traitorous to the loyal fans. Afterall, these movies look like they'll be playing around 2:00am and later ... something tells me the majority of TCM's loyal fan base will have all gone nighty-night by then. Second, Mr. Zombie is in fact a filmmaker and a self-proclaimed film fanatic, so he's not a simply random choice as a host. Third, consider the line-up .... Mudhoney, Night of the Living Dead, Plan 9, Bride of the Monster ... these are certainly classics in their own right and are best hosted by someone like Rob Zombie, if not Elvira.
So, instead of blowing off the already loyal fans, it seems to me that TCM is embracing other people's deifnitions of "classic" and placing the series in a logical place. In fact, I'd go as far as to say that they've made an effort NOT to be traitorous to the loyals.