In my experience, this depends on the community size.
In a small community, "platforming" assholes (rather than "deplatforming" them) may act to retain the assholes as users (where they would otherwise leave for lack of platform); and then those asshole-users, since they're already there, may also interact in other, non-quarantined subforums on the site — to other users' detriment.
In a large community (society, really), e.g. Reddit or Twitter, the asshole-users are going to stick around either way, since people have multiple interests, and the site likely already gives them many other things they want besides just "a plaform to talk about their asshole opinions on." They weren't there primarily to be assholes; they just are assholes, but are going to stick around either way.
So, for large sites, the only real decision you're making by quarantining vs banning a certain sub-community that's full of assholes (rather than doing active moderation of certain discussion topics, regardless of where they occur) is whether the asshole-users' conversations mostly end up occurring in the quarantined forum, or spread out across the rest of the site where they can't be hidden.
It's a bit like prostitution regulation. Prostitution is going to happen in a city no matter what; it's just a question of whether such activity is "legible" or "illegible" to city government. Some cities choose to have an explicitly-designated red-light district and licensing for sex workers; these cities at least ensure that any activity associated with prostitution — e.g. human trafficking, gang violence, etc — occurs mostly within that district, where police presence can be focused. Most cities, though, choose to "protect their image" by having no such district. This option does not result in less prostitution; it only hides it throughout the city, making police investigation of crimes related to sex work much less likely to be reported, and much more difficult to investigate.