A horrible theatrical adaption might not ruin the source material, but it's still a disgrace to the franchise and a slap in the face to the fans of the book. The Last Airbender movie didn't ruin the show it was based on, but everyone hates it because it squandered its inherent potential, made characters OOC fools of themselves, and otherwise failed to re-create anything that drew fans to the original series in the first place.
Supporting the film, for many of us, is basically condoning the butchery of the source material to create a sub-par adaptation that relies on the popularity of a book to make any sort of profit. I'll use my time, money, and effort to support a movie if it's
good, and not simply based on a novel that I like.