Hull to Beverley's ~16 minutes against 33 for Nottingham to Mansfield, and dealing with a London service running every two hours rather than twice per hour; presumably cutting Beverley extensions wouldn't save any units.
It looks like some of the Hull Trains services have enough turnaround time at Hull that they could be sent to Beverley (a09:58-d10:33, 14:17-15:08, 16:25-17:08), although there's a 10:00 Hull to Bridlington and 14:21 and 16:21 to Scarborough which mean doing so probably wouldn't be picking up many passengers at Hull.
Most of the HT services to/from Beverley are at the start or end of the day's service – the only exception is the 1548 ex Kings Cross, which arrives into Beverley at 1843, then runs light back to Hull to form the 1910 to Kings Cross – so no, it doesn't increase the number of units needed.
Extending the daytime services to Beverley would add in complications with fitting around the Northern services – and would also take a lot of slack out of the timetable, which is needed for crew breaks, recovery time, cleaning and restocking, etc. The likely patronage to/from Beverley outside those peak services is small and is easily catered for with the half-hourly Northern service, and this allows the main service to be run reliably.
It's hard to see how you could use the same model could be applied to the Robin Hood line. The only trains to London that start their passenger diagram at Nottingham are the 0549, 0611 and 0649 – would there really be the demand to run an extra train from Worksop even to form the 0649 (so departing well before 0600)?