For me this depends on the size of the book, as others have mentioned. I prefer larger gaming books to be hardcover because they'll last longer, particularly if they see heavy use (which most good monster books do). Smaller collections -- Armies of the Abyss, Jade Dragons & Hungry Ghosts, Monsters of Faerun, etc. -- don't need to be hardcover.
At the same time, as long as the content is worthwhile I don't really mind how a book is bound. AEG's Mercenaries, for example, is a 256 page softcover book -- despite being more than large enough to justify being hardbound -- but that didn't stop me from buying it.
I really disliked the old binder system, mainly because the pages tore out and integrating new material was a pain in the butt -- especially with the smaller expansions, where an "M" monster might be followed by an "O" monster, meaning something would be out of order wherever you chose to put it.