Consider also the runway state. You say reverse thrust is not that effective below 80 knots... in the aquaplane / ice / contaminated runway it is more or less all you have got. Our SOP is to keep reverse idle until taxy speed as it takes around four seconds to spool up to full reverse from this contition (as opposed to 12 seconds if you have retracted them fully). This was reinforced by an incident where the reversers were stowed on a wet runway at 60 knots IAS (10 knot tailwind) and heavy rubber deposits at the upwind end put the brakes in a virtual "black ice" situation. It took so long to get the reversers back out they came very close to the grass.