You say ‘real’ matches as if skirmish is a ranked mode. It’s not.
Could it potentially be abused? Yes. But the cases of this occurring would be outliers at most.
There is no vote to kick option, so the party could not boot the other hunter from the party.
In order for them to win, their friend(s) must lose. This is no different than how things were back before online play.
The only method of abuse that could feasibly occur is trash talking. Which will happen anyways with hunter teams of three, or post game with teams of four. Some people are just assholes. This doesn’t solve that issue and never will.
The only method of exploitation would be farming mastery. Which can be done in a custom game without a random ruining it. So, this isn’t a good reason either.
Leaderboards are bunk anyways, and besides, if a team tries to screw over that one other player, all three of them have to take a loss as well. This would be a valid argument if say, teams were of equal size, but if a team of four were to try to screw over another player with one of them as a monster, only one of them would benefit, while three would suffer negatives.
The three might abandon the other player? Happens anyways, teams of three hunters are allowed.
I can see restricting the monster player in a ranked mode though. The monster could feed losses to the hunters, and rotate out. But this isn’t a ranked mode. This is skirmish. There is no reasonable benefit to take advantage of the system in skirmish.
If you’re the type of person that doesn’t need reasonable benefit you are also probably already ruining peoples day, and not being able to play as monster in a party is not effecting that at all.
I can find no discernible reason for monster play in a party, in skirmish, to be disallowed. UNLESS they are intending to turn skirmish mode into the ranked mode.
This restriction is akin to saying “No you can’t have that rocket ship toy because you’d use it as a boat.” It’s silly, and disappointing.
Edit: Behold, for I am a powerful necromancer! Maybe I should read the date of the last post before I say anything. =/