That'd introduce a fall hazard as you're then creating a situation in which an open door usually has a ramp, but sometimes it does not. It doesn't take much for someone to lose track of the situation (e.g. guard getting asked a question by a passenger) and then falling between train and platform because the step has retracted already.
Fair enough.
I was thinking it would be OK because it's only being used by the guard at that point, but of course it's quite common for latecomers to jump in using the door the guard has open.
I have to say though, that given we allow non-FLIRTs on the network and indeed FLIRTs on curved platforms where the step isn't enough to fill the gap, I thought the steps were to permit easier access especially for wheelchairs, pushchairs and the like, not because a gap beween a train and a platform is now considered too dangerous to contemplate.
But I suppose if someone is expecting it to be there and it isn't...