As an example, look at the Kings Fund bed. There you have the best of both worlds (lockable "steering", or free, as the occasion requires). Free steering (castors) on all wheels simply leads to the trolley having a mind of its own (eg,
when encountering camber, "pot-holes", and all the rest), calling for almost constant extra effort by the "driver". Good exercise for the abdominals, though. By design, four castors implies two operators (one at each end). Anyway, I rest my case (as someone who has probably put in more "trolley miles" than most on this forum, I should imagine).