This should not even be controversial. Iran is an important player in the Syrian civil war, being a close ally to one side and which reportedly has its armed forces actively fighting in the conflict. The whole idea of these conferences is to get all the relevant parties to a conflict together to see if they can find a way to stop it that everyone can agree to. Iran is clearly a "relevant party" and if you leave them out of the discussions it means that any solution that is reached will not take Iranian interest into account, which greatly increases the chances that the deal won't work.
The reason people advocate keeping Iran out is because its government is awful. Yes, in a lot of ways Iran is ruled by terrible people. But the reality is that the world is filled with countries run by terrible people. So unless you are going to refuse to speak to most foreign governments, being terrible should not be a good enough reason to exclude a foreign government from talks. And seriously, any talks that involve Saudi Arabia cannot possibly exclude other countries for being terrible.
The reason people advocate keeping Iran out is because its government is awful. Yes, in a lot of ways Iran is ruled by terrible people. But the reality is that the world is filled with countries run by terrible people. So unless you are going to refuse to speak to most foreign governments, being terrible should not be a good enough reason to exclude a foreign government from talks. And seriously, any talks that involve Saudi Arabia cannot possibly exclude other countries for being terrible.
