During our WDW visit at the beginning of January, our adult DD joined us for the first part of the trip (arrival Wednesday, departure Sunday--5 days and 4 nights). We had an old Park Hopper with one day admission left and both DW and I have annual passes. DD used the one day on day of arrival (Wednesday) and purchased UMP for 3 days since she was departing early on Sunday morning. She had no problem leaving off both first and last days. If the CM had not been willing to drop both first and last day, it would have been less expensive to purchase a regular 4-day park hopper and save the extra day for future use. The park hopper cannot be purchased for less than 4 days, but those days never expire so you can always use it during a later visit. The UMP expires on the selected day during that visit, so there is no carryover if you don't make a park visit on one of your days. On the other hand, the UMP includes water parks and other features not on the standard Park Hopper, but visiting a water park during the first week of January this year would have required more fortitude than we have (it never reached 70 deg during the week we were in Florida; meanwhile, it was in the high 50s in Chicagoland).
The CM who checked us in never hesitated regarding the number of days we wanted for the UMP.
Ralph