You'd be surprised at how much it benefits YOU when you show up at your originally rostered report time and find out that something has changed which, due to your signing on at the rostered time, you will NOT be able to do!
I think Sir KDM has a very good point: if NONE of us acknowledge roster changes through Crew Direct there will not be enough man hours available for Screw Control to call everyone!
However, based on how I've seen our conditions erode over the years due to our own selfishness, I doubt a significant number of crewmembers will follow the "do not acknowledge" hint.