Here is a quote from him: "Blocking a world leader from Twitter or removing their controversial Tweets would hide important information people should be able to see and debate,” [...] “It would also not silence that leader, but it would certainly hamper necessary discussion around their words and actions.”
He apparently feels there will be wider discussion from all parts of the political spectrum if things are left as they are. Should Trump move to a platform where he has more control over the format, it might just make it easier for him to suppress or obfuscate dissent.
I don't know if Dorsey is making the right call here, but he has clearly thought about it a lot, and discussed those thoughts publicly.