Being asked to join a PUG because there arent enough people at the moment and then not being picked because a couple of aces show up in the middle of the pick blows hard. Close the door when picking starts. If "Ace Legion dude" shows up in lobby during pick tough luck, start game and no trying to get him in PUG channel.
Outside of that I understand the need to win and no one wants to receive a shelacking, playing just to play is what PUBs are for. The good players get picked first thats the way it is, perhaps a tier system for games. First tier are experienced above average players and second tier seasoned but not as experienced ( 3rd tier just needs to grind it out in the PUBs). Either have different PUGs for tiers or have a required amount of tiers per team. I know that might edge out some really good players depending on the amount of people that show up for pick. It would also get messy defining who is on what tier.
Completely agree with you. Once picks have started, the channel should be locked. Besides being cheap, having a good player jumping in and getting picked late usually ruins the PUG.