Well, i think that it would be great. Let me use a real football example to compare. Let's imagine today is United x City. Mourinho has chosen a starting eleven and so did Guardiola. Their initial squads are now closed and they can't change their players. In the other hand, they can still do somethings:
1) Change the formation. Not the starting eleven. Mourinho can still decide if Martial will be a lone striker with Rashford on the left or the other way around. Thinking about T11: The players that watch the match could have 5 or 10 minutes to change their formation after the release of the squads. However, we wouldn't still be able to see our opponents formation. Only their starting players. If a guy have the players to change a 4-4-2 to a 4-4-1-1, you wouldn't know which one it would be before the game really started, as we all know, most of the times the teams try to hide what they plan to do.
2) Change their tactics. After the squads relase, Guardiola sees that United is playing with Pogba, Herrera, Mkhitaryan, Mata and Rooney on the midfield. He would probably assume that United is going to be offensive, as there are no holding midfielders, and therefore he would try to shape his team with a defensive mentality, trying to focus on counter attacks. Thinking about T11: The starting eleven from your opponent is available. You can see if your opponent has chosen to play with a holding midfielder or to play with one more striker, so before the match start you can still change if you are going to play with offensive or defensive mentality, for example.
3) Those changes before the match starts would have to be quick. Maybe 5 or 10 minutes like mentioned above.