Braking effectiveness is reduced as much as 60% without speedbrake deployment.
Rapid deployment of speedbrakes is vital and it's main job is to reduce lift. The aircraft can very quickly become airborne again if speedbrakes do not deploy on main gear touchdown. Auto deployment can also be inhibited by touching down with thrust on which also has the same effect of increasing the risk of a bounce.
You do not want to train/rely on the application of reverse to raise the speedbrakes, as pulling the reversers commits you to a landing and they can also be deployed below 10' RA.