An ill-fitting helmet does not protect one's head.
If a plastic cereal bowl can be dropped from 3m (or whatever CPSC's standard is), should I rubber-band it to my dome?

And road helmets aren't just "more aerodynamic" than MTB helmets. Usually they're the same as their counterparts, just with a visor and different paint.
If there are differences, they are things like different sized and shaped vents (road bikes move at an average of 20mph, MTB's move at an average of 12mph) and slightly better crash resistance, so that if a rider crashes halfway into a ride, he/she might be able to finish with a helmet in one-piece (and then throw it away).