Thanks to all for your responses. One argument not yet mentioned (I think?) in favour of pre-soaking film is to ensure even development across the whole film area when development time is short (i.e. with a fast-acting developer).
For FB paper, MarkL has given a pretty good reason FOR pre-soaking, which I can identify with as my darkroom sessions are limited too. The chief reason for NOT pre-soaking FB paper would seem to be progressive dilution of the developer, leading to inconsistent prints.
I still don't have any feel for how quickly chemicals diffuse through a pre-soaked paper base. Anyone got any insights?