Neither a movie nor a miniseries will be as good as the book, so following your logic they should just not bother with it at all.
I mean, why else make an adaptation if not to bring the story to new audiences? Sure, it'll be kinda cool for us fans to see our favorite story acted out, but I'm personally much more excited to be able to share this amazing story with people I know who would never pick up a comic book or watch a miniseries.
Right now I work at a video store, and while I dont know if I'll still work there when Watchmen's out on DVD, but if I am I cant wait to be able to recommend the movie to people and know they're gonna go home and watch it and probably enjoy it. We almost never carry minseries', and when we do no one rents them.
Additionally, I love seeing trailers for comic book movies in the theater. Last night at Iron Man I got to hear all the reactions to the Dark Knight and Incredible Hulk trailers, and it was cool. Wont get that with a miniseries.