Marauders in a bunker versus 5 Roaches:
It takes 5 seconds for 5 roaches to kill a bunker, in 5 seconds the marauder will attack 4 times (3.33 but given marauder has a high range I'll say 4) which will do 100 damage not even killing 1 roach. Once the bunker is dead the roaches take 4 seconds per marauder while the 2 marauders will do 100 damage not even killing one roach in 4 seconds.
Roaches are cheaper, can be produced faster, same speed, do 4 less damage but have 20 more health, the only thing going bad for it is that it has 2 less range than a Marauder. I think people overestimate the Marauder, even versus armored it barely out damages a marine.