Infantry should have the option to attach themselves to vehicles in emergency "GTFO" type situations.
Several downsides could be put in place for riding on a vehicle by a means not originally intended (such as falling off, loosing grip, rolling around, etc) but if nothing else, it usually involves riding on the outside of a vehicle and, thus, not being protected from hostile fire, debris, or crashes.
See att. photos {F17415} {F17416} {F17417} {F17418}