well spot i dont know if you are pro captain or pro player but in perspective i think on this team alot of the other players picked to practice for playoffs/districts/sectionals would agree that leaving Player A off shouldn't have happened even from the group that practices for playoffs. Why leave a strong player off the group of practice partners even if you weren't going to play him/her?
Let me add a few things:
First, I don't believe in 'pro captain' or 'pro player'. It should be 'pro team'.
Second, in general, the captain should do what he/she thinks is best for the team. Sometimes there will be disagreement, and one or more players will a different opinion. At the end of the day the captain needs to make the call. Now if as a player you think the captain is knowingly acting against the best interests of the team, then you have a legitimate beef. But on the other hand if it's just that you disagree with the captain about what's best for the team, but the captain is doing what he/she thinks is best, then you have no reason to complain. Not everyone is always going to agree about what's best, and ultimately the captain needs to make the decision.
As an example, when I pick playoff lineups, others might disagree with my evaluation of what the strongest lineup is. I don't consider just head-to-head results. I also consider how people have done against quality competition during the season. Also how well they handle pressure - playing a playoff match is different to a regular season match. And in many cases I know our opponents and so am also influenced by the kind of players they have, the style they play, and the most likely matchups.
Sometimes teammates will disagree with my choices, but they all know that my intentions are always to do what's best for the team, so they respect my decision. And I have a good track record so I am more often right than wrong.
One other thing you said though was that the captain had a 'playoff practice group' that does not include everyone on the team... that I don't agree with. We value everyone on our team and everyone is encouraged to practice together whether they are anticipated to be in the lineup or not.