I think guests scare away ppl because they dominate the conversation. That's an assumption based off my personal experiences because they scare me away when I see the type of conversation going on when they're in there. It'd be great if everyone would block them but they don't and most ppl get trolled by them. When that happens, even when I have them blocked, I have to see the other person responding to being trolled which makes me not want to talk. I try to start a conversation but the person being trolled is already caught up in it and feels the urge to always respond. I tell them to block them and they rationalize it by saying, "No wait, I just want to see how stupid this person really is".
It's unreasonable to expect graphs to back up opinions unless that's the requirement for any opinion to have any weight, ie. you'd have to get graphs showing the site activity and joining members when guests are allowed to back up your opinion that guests don't scare away members. For sure, it would take out the bias possible and ideally, that would be the requirement of any debate, that if you have an opinion you need empirical evidence. But that's a lot of work.
It says something about the person and the type of conversation they are looking for when they don't want to go through the very small barrier of entry of free registration. I think it says that they don't respect the conversation and require the maximum anonymity possible to feel safe acting the way they do.
I don't think banning ppl helps either and it exacerbates the guests on issue because ppl who are banned get all their friends to come in and troll the chatroom in hatred and anger. But I think that's a different issue.
The amount of protection required to maintain civil conversation in any forum is a representation of how empathetic the main proponent of their forum interests is. In a perfect world, no one is banned, everyone excercises ostracizm, and everyone enjoys cotton candy.