I decided I needed the speed sensor for trail riding MTB because there is no way that the GPS can track the tight turns. Also, when you climb and descend over a small hill, the GPS will only see the horizontal movement component and not the up and down. Two of my friends went for the same 20 mile trail ride and their distances were over one mile apart. This is both due to GPS error and auto-pause errors. The wheel sensor ends the auto-pause errors.
Is one mile off over 20 miles good enough? No, not for me. No way. I use Strava and I don't want the GPS to say I went 19.0 MPH when I went 20.0.
For a road bike I think it is much less important - though a GPS will still clip corners when you turn, so I just got one for my road bike.