There is zero, none, null, nada, etc reason for anybody to tell any other person that they are playing with people they know. It has nothing to do with being secretive. It has nothing to do with wanting to win or lose.
So there's no reason to hide it. "Just so you know, we're on vent." That's an extremely crucial piece of information to the people on the opposite side who probably AREN'T ON VENT. Of course you don't want to tell them this, you have a massive advantage going in that they don't. They might leave. You don't want that.
Who I know should have no bearing on you. How you play as a team depends on you and every other person on that team - knowing them before hand does not change this
It sure as hell does! I play VERY differently if I know that hero next to me is controlled by someone I know. I will be more aggressive because I know they will gank with me, and they will cover me as I flee and won't bail at the first opportunity. You are far more cautious if you don't know whether that Sedna is going to heal you or not. Assuming she will has bitten me and innumerable others.
How anyone can deny knowing the mannerisms of the people on their team gives them an advantage is insane to me.
How badly you get beat has nothing to do with the other team.
...What? If the other team is equal in skill (and not using vent) you won't be badly beaten. If the other team is equal in skill, and a premade, and using vent then you will get rolled.
It is on you to decide what game you want to play in - the host and anybody already joined has no responsibility to tell you that they are noobs are gods
And witholding that information prevents people from making an informed decision - but that's what you wanted, right?