I think there's other points to consider when looking at the rosters of players.
I would think that when the coaches are forming the pool/team, they in fact have to break the players into at least 4 groups, keepers, defenders, mids, and forwards. With significant evaluation time, ie, club seasons, coaches can learn which players can play multiple positions, but in the yearly ODP shortened duration, be it 1, 2, 3 tryout sessions these coaches may not be able to determine that. So when you look at the group in this manner, looking for a roster of 18, I don't think it would be prudent to select all mids and forwards and try to create defenders. At the tryout that I observed, somewhere during the session, the players were asked what their position was. GKs are pretty obvious. Relatively small number of hands went up for defenders and forwards and about 20 or more hands went up as mids. With 11 players on the field, 1 keeper, 3 or 4 defenders, 5 or 4 mids, and normally 2 forwards. Looking at subs, another keeper, maybe 2 defenders, 2 forwards, only leaves room for probably 2 mids. This is where the extended club season kicks in because at this level, you reasonably have players that can play multiple positions. So in the cases of only selecting 1 team, you are left to select the team from these smaller pools, and therefore, some interesting selections take place.
Add to this the dynamic of asking the "players" where they play and it gets even more interesting. A player may feel that he is a mid, and therefore when asked, he says mid, but plays as a forward or defender with the club team. So this player that feels he's a mid, may not fully understand that mid is not the best position for him.
I do agree with the coach communication point, to a certain extent. I'd like to add that with different coaches comes different styles of play. I personally think exposure to different styles of play makes the player a more rounded player. There are players that fit in the 4-4-2 for example, but not necessarily fit well in the 3-5-2. If a coach has a particular formation/style of play, then he probably tries to find players to fit that style. So while consistency in coaches would/could add consistency to the rosters, changing coaches and styles may in fact round out the players development.
Just a couple more points to consider.
