Thisisyesterday "Most" Nurseries don't do this. In the course of a year, I go into probably about 50 Nurseries. There is one of these that makes people remove shoes. Hardly 'most'.
Re safety - slippers don't tend to have grips - there are rooms which will not have plush carpets on the floor (I'm suggesting the hall and the toilets, even if corridors are carpeted) that then becomes a sliding heaven for children (have you not seen Peter Kay talking about dcs at weddings ?). That's the only hazard I can see for people in sensible slippers, but have you seen some of the clippity clopitty heeled ones some children wear. Not all dcs are the dcs of sensible MNers you know.
Oh, and what about going into the toilets.... wet floors etc..... have you been in a Primary school toilet towards the end of a day recently ?