I wouldn't have an issue with someone rejoining. However, adding someone later in the game could be exploited. You go into a match with a set lineup. Just like in real hockey, you shouldn't be able add someone mid-game that's not on that game's roster. This would especially make a difference when it comes to club finals.
I think it would be great if someone could reconnect. The thing with that is we can't tell if they purposely disconnected or not. What if someone had to take off for a bit to handle something else? I get that emergencies come up, but you shouldn't be able to leave the game and return whenever you want. What if a team is down and all the humans but one leave so that AI can take over? 4 AI players can be pretty challenging for some to play against. It really wouldn't be fair if you let them get you back in the game and then everyone rejoins. This is something that would most likely be exploited.