Without sliders the 'normal' erg needs utilizes straight power developed from the deep-compression at the catch all the way through to the finish. In my opinion, the finish on a normal erg needs to be dramatically magnified or over-emphasized to force the flywheel to spin at at higher rate going into the next stroke. This prevents the flywheel from slowing down as much on the recovery back to the catch and in turn less power is needed to pick the flighwheel's speed up (See picture below). The way I see it, the force applied at the start and end of the stroke compared to the recovery will give the power reading (watts or speed/500m). The true effort in erging without sliders is to develop force against the flywheel but also the ability to change direction (momentum) of your body-weight between the drive phase and recovery phase, which isn't used in on-water rowing nearly as much.
Pilfered from facebook (D O'F photo)
Even though I have only done a half dozen workouts and a few erg tests on sliders, I prefer them. In my mind, erging with sliders is more like on-water rowing and that is the main reason for training indoors. Also, I feel there is a decreased chance of injury using sliders.