I think it's an individual thing. My guys really liked the pins, but didn't care at all about the autographs. Pins and autographs are another way to make memories/have an interactive experience at the World.
Might depend on how old your children are, how often you've been/are going, etc. With my guys, we didn't do the pins/autograph book until they were a little older, but I know many people on here do it with their kids from very, very young. Nutshell: If you feel like checking it out, try it! If not, there's tons of other stuff to do/see and your kids will probably never miss it if you don't point it out. Hope you have a great trip!