The secret to PQD is not thinning the least valuable elites, although it's a benefit. The most important part is to stop US elites from booking *A tickets with other airlines and earning UA status without UA seeing any money. PQD effectively encourages codeshare or JV flying instead of buying the cheapest/easiest seat on a competitor website.

I know a few J fliers that have starting routing through UA hubs to get on codeshares instead of flying direct. Non codeshare partner flights are effectively priced out of market (UA website) or non PQD earning (competitor website). Hence the drive to book codeshares or UA flights on united.com.