I know it might be a little late to post my opinions on this, but I didn't know it wasn't possible to switch players between matches until just a while ago.
I personally think you should be able to swap out team members between matches in rounds. What thepyrosgod said is one of the first things that can go wrong, which is a good point.
The tournament is a team effort, but you can only use 50% of your team in a round? We, as a team, obviously like to win, but in the end it's all about fun. We'd like to give the opportunity to everyone in my team to play, but by locking 4 players to a round this limits me in doing so. We're kind off forced to choose between having fun and playing to win, while if we can switch players between matches it can be balanced. Our team wants to try and win games, if we make it farther we can have more fun. So if we could switch between games, we would always start with our strongest line-up, then we can gauge if we need to continue playing with the strongest team or if we can send out the other guys if we are clearly better than the opponent.
Let's say my team sends out the strongest team we have in the first round, but we have stiff competition. We keep playing with our strongest line-up, because we want to win, however we end up losing. Now we're only one loss away from being eliminated, but only 4 of us have played. We now have to choose between sending in the other 4, so they can play a match too, or still sending in our strongest team so we don't get kicked out immediately. Switching members also gives teams more ability to adapt and change strategy between matches, it's always nice to give teams more options, right?