The thing about 'practicing' in QP is that you have to be the only one on your team doing it. Otherwise, you are blatantly working with a character you cannot use very well, handicaping your team. If 2 other people are doing the same thing your doing, then your team is basically not doing what it needs to to survive. And if the other team isn't, if they're just playing regularly, your whole team is fucked. Meaning you're going to get killed before you have a chance to do any practicing.
And A lot of the heroes just aren't going to make any sense by yourself. Winston needs to coordinate with his team to dive properly. Ana needs to people to say when they have their ults ready to plan ahead. Sombra needs her team to move in when she EMP's. Those are probably the most critical heroes that need a coordinating team.
The best practice sessions you can have is when you aren't necessary. If your team is steamrolling them so hard that they can essentially beat them 5v6, then that's the time you whip out the Widowmaker and take your potshots at the enemy team struggling.