It's not that the passengers are few enough for a two car to be sufficient, it's that this is what people are forced to squeeze into (or stand on the platform watching helplessly while unable to board, how often does that happen in London with the next train an hour away?). More people would travel if they weren't forced to wedge into a two car DMU. It wasn't that long ago that Manchester Airport to Blackpool services would have been so formed (possibly with a dreaded 142), now they are six car EMUs and are well-loaded. In East Anglia services that were once single 153s are now operated by FLIRTs with twice the capacity, resulting in a growth in patronage.
Twice the capacity?
The 153s had somewhere between 59 and 75 seats according to wikipedia, and only the 59-seat version met PRM requirements.
The FLIRTs have 202+27=229 (4-car) or 144+23=167 seats (3-car).
so that is more like triple to quadruple the capacity, and it is needed on the routes now.
More capacity leads to more passengers, especially when combined with other improvements, like the increased reliability and comfort at GA.
EDIT: GA didn't just increase the seats per train, but also the total number of trains in the regional fleet