on about every moped i've ever been on:
1978 puch maxi (single rider seat/luggage rack)
197? motoguzzi robin (single rider seat )
1980 garelli super sport (long seat)
1989 derbi variant sport (long seat)
1992 tomos golden bullet ttlx (long seat)
2000 tomos targa lx (long seat)
i've ridden a passenger or as a passenger and i'm 6'3 180 lbs myself and the other person was either about the same or a girl of varying weight of what 110-130 lbs? so from my experience they can handle it. granted they run best without them and that's normally how we ride. but if you have to do it sometimes i don't think it'll hurt.