Why these? I find them slightly easier to step in (compared to etrier style aiders) initially, because the steps aren't offset. In other words, you can step left foot into first step or right foot first and it makes no sakes. The etrier ones, if the first step is on the left, you either have to step with your left foot first or flip the aider around. Also, they are generally regarded as easier to learn to aid with, although slightly heavier than etriers.
Ant, well spotted on the pinching issue. Yates and Fish does them with a spreader bar, basically a piece of PVC pipe fitted over the top step, to prevent exactly that pinching of the top steps. People also add their own spreader bars to the Metolius ones.