I am in the process of turning my novel into a comic and honestly, if I were to be approached by one of those companies to turn it into an anime I would most likely say "yes". I've always loved the idea of my story becoming an animation (I don't have the skills for it at this time) and if it were to get big enough to catch the attention of these companies, that would be like the "hey look ma I made it" feeling.
I know it could potentially be drug through the mud, so if that ever happens I will be sure that my contract or whatever states something along the lines of "the story/characters cannot be changed and the original creator has final say on any changes...etc" and have a good lawyer to make sure things run smoothly. I wouldn't sign anything unless my lawyer has read through the contracts, explains everything to me, and is sure I won't be getting scammed or lose the rights to my story.