you can apply all the pressure without driving the car fast you can do it when stationary!!
you don't have to be driving fast to check them nor do you have to brake stupidly hard
during an MOT the rollers aren't been used constantly and you can easily get a car on the rollers while a car is been inspected underneath on a ramp!
plus you have load balancing on the rollers too
so there is no excuse
Now you're moving into another problem which is the fixed priced repairs
£229 for second year and £49 for the BFC they are done as quickly as possible the road test is part of the service so why make the extra step to test on the rollers if you have to carry out a road test anyway.
It also depends how the workshop is set up for example one workshop I worked in had the MOT station set up as a drive in drive out with the 4 poster and brake rollers all in a line with an exit on the other side the cars where then lined up and MOT'd one after another this meant to use the rollers for a non MOT car would take 15-20 mins to get the workshop clear for a non MOT test.
Another Main dealer I worked at the MOT's where done at there sister site as they had 5 garages (different Brands) within a 5 mile radius so the garages where picked up MOT'd and brought back to the site.
And As I said before Hard braking is not just for testing the brakes. you are looking for how it responds steering wheel grabbing to one side ABS cutting in pedal feel at different pressures
I'm not saying to drive it like you stole it but a test drive is not a leisurely cruise around the block and the vehicles are tested to make sure they work correctly under all circumstances.