Very bad idea.... When the bike goes down, the weight of it holds the passenger's elbows and arms in one position into the grinding abrasive surface of the roadway and the passenger then rides out the scraping slide until the bike stops. TYpically in a crash, the bike slides much farther the than the rider who got pitched off.
There's a reason that motorcycles don't have seatbelts
2004 Rebel 250, 2003 BMW K1200GT (roadburner), 2004 BMW R1200GS(all purpose),
1973 Norton Interstate (in a box in the basement)
1968 Triumph Bonneville