While you did say buses specifically rather than coaches, I will mention that the X5 Oxford-Cambridge service has several set-down only stops within Cambridge city centre. The stops after Northampton Street up to the terminus at Parkside are all set-down only (it's stated on the stop signs themselves that this is the case).
The inbound X5 stop on Maid's Causeway in Cambridge *is* available for pickup (I use the stop for the 3 most days) and the X5 literature on the shelter makes no mention of being set down only, coming with a full timetable and buses shown on the Real Time Info screen. Not that anybody ever gets on anyway as it only takes you round the block!
The penutinate stop on East Road opposite the Grafton Centre is set down only, as is Madingley Road Park and Ride on the way in (to save the bus detouring if nobody wants to get off)