Sounds a bit dodgy to me.....
What if there was an APU fire? Or any number of problems could come up. We have often used a 'dispatch crew'. Once had a crew arriving on a positioning sector to take over operation of an aircraft. Serious ATC flow restrictions that day, but luckly they had an on-time slot. Unluckly their position flight didn't. They were due to arrive -30 from departure. We had another crew sitting around, who while not able to operate , preaperd the aircraft, fuelled the aircraft (whilst talking to the operating skipper on his mobile, re wx and notams and how much he wanted). So whilst the crew waited for the bags to arrive and got through customs and out to the aircraft, the other crew had done everything for them, including boarding the passenegers. they got on, went through their checklists and then bu**erd off on time.
I have no problem with that, but not to sure on passengers boarding without any flight deck in attendance.