What exactly are the pros? Being able to watch a game? That's about it because once it's over, you have to join a new match anyways which means being in a lobby. I would say that the cons easily out way the pros. Having someone and spectate the game would lead to considerable ghosting abuse. It would be like having another team mate. They could fly around, tell you where people are, what gear they have, if they just finished a fight and if they are weak. In UHC, they could fly into the ground and tell you where diamonds and caves are. That is just game related cons.
On the server side, there are costs and downsides as well. In order to allow spectators in matches, you would need to increase the amount of spaces each game would hold. Each spot in a game costs money so this would increase how much Cubecraft would have to pay just so people can ghost for their friends. This would also create some lag because lots of people would be in one game.
Sorry, but this would not be a good thing to add. -17