The main point of retention is to ensure that contact is confident and consistent.

Using more muscle groups doesn't improve efficiency, it takes energy and oxygen away from the muscle groups that are good at pedaling, and the imbalanced stroke can increase the prevalence of things like knee and lower back pain.
It does provide increased peak torque, but this is relevant far less often than sometimes claimed. You have to be wildly overgeared for the upstroke to be necessary; even 60rpm mash-cruising is something that a no-pulling stroke can handle entirely smoothly.

(Similarly, the need for retention at high RPM is also very overstated. On a bike with a freewheel I'd go as far as to say it's completely irrelevant. I regularly see people saying things like "you simply wouldn't be able spin 120rpm on platforms", but I've never noticed any difficulties doing such things.)