I still cannot conceive how people can say that anything in QP qualifies as 'practice'.
Even all your doing is trying out your abilities, the way you use them in QP is not very applicable to how you use them in a serious match in the event that 3 people on your team are not playing 'seriously'. I just got railroaded by a fucking Torb stack. I don't even mean a team of torbs, 2 torbs, with a Reinhardt shielding them and a Zenyatta throwing shit at you, plus 2 others who were doing fuck all. I tried using Genji, but 'practice' or not, he's going to get shot down the moment he goes into the room. Not because he's not useful in that situation, but because the other players are doing fuck all. Soldier is peppering them with fire from far away while a mercy standy by upping his damage, as if that'll do anything. A D.Va is doing nothing of worth. A Widow is just sitting there, endlessly sniping the turrets that endlessly get rebuild. The only kind of push we had is when I decided to nuke them as D.Va from above, so the Reinshield wouldn't protect anyone, which you'd think would be my teammates cues to just take everything.
The only real practice mode is practice against the AI, because they'll atleast be doing what they can to beat the other team with the same competence that the other team is trying to beat them. Otherwise, Comp IS the practice with any particular character in tow, because that's the only place where people will not let the entire group fall to shambles