I've considered this question.
When I was researching the Creation Book, I felt the Society's complaint about changing scientific conclusions was just as hypocritical. That is, where the Creation Book tries to undermine our confidence in science by pointing out how many of their own ideas they've shot down over the years, I kept thinking of the "light gets brighter" doctrine and how similar they are. Why can't the light get brighter for science as well? Especially since scientists don't claim to be "magically" inspired by outside forces.
NOTE: I already know creationists and evolutionists disagree. That's fine. Please, let's not argue. I'm only citing my impressions of the Society's argument in favor of creationism. Please don't hit or yell.
Anyway, that got me thinking more about it. The thing is that the Society claims divine inspiration from Jesus, except it seems that Jesus...for some reason I can't understand...has to be all sneaky about it. Even they admit they don't get phone calls from Heaven and that he never pops in and calls them all in for a board meeting, right? So how, exactly, is he guiding them toward this "new light?" Why is he being so coy? And how can they prove--even to themselves--that they are being divinely inspired in any way? I could claim to be divinely inspired and start a cult if I wanted. "Obey me or the god of phlegm will slime you to death!" But that wouldn't make it true.
What everybody else said about holding them to a higher standard goes too. If everyone teaching false wisdom are false Christians, then that means that the WT was false Christianity the day before they revealed their new light. And the new light never stops. So when in their history, if ever, will they fulfill this requirement of being truly Christian? They are holding themselves, and so-called Christendom, to a double standard. They go way too easy on themselves for a group professing divine inspiration, yet treat everyone else with the same skepticism they should be treating themselves. After all, you'd think they would at least need to prove to themselves that they are divinely inspired, wouldn't you?
IsaacJ