"Mid or feed" trolls are more likely to not care about a potential trade in champ select, so they will almost never buy champions they don't intend to play often. (after they have taken care of their own champion pool)
Which means that they have higher chances of getting champions that are actually useful and fun in ARAM, like long range mages,etc. All because they can't random a champion outside their owned ones. Think of it as having one of those ARAM smurfs, but not on purpose but due to having a bad mind-set.
Meanwhile, players who buy champions because they are considerate and want to cover a potential trade will get punished, because they will get that Olaf or that X or Y that will make them stay under turret for the next 20 minutes and suffer.