No, I understand: it's the idea that keeping them in the spotlight means we can keep tabs on them easier. I can see the appeal of that solution, but here's the problem: some people will legitimately go to /r/The_Donald to figure out what these people are thinking. Researchers, political operatives, etc. might find this very convenient. But the other effect this has is that (a) it gives them legitimacy (look! One of the larger subreddits on Reddit is an alt right haven!), and (b) most people aren't critical thinkers. They will see a political meme that mildly aligns with their beliefs, chuckle, move on. Until they see another, and another. Eventually, they'll notice a pattern: these memes are coming from TD, so they subscribe. Next thing you know, they are only getting their "news" from the TD memes, and suddenly they live in a political thought bubble getting further indoctrinated. That's what will happen with 99.9% of the people who end up on a subreddit like The_Donald, whether you like it or not. So in my estimation, the benefit of keeping TD around (easy visibility into the thought process of these individuals) is greatly outweighed by the drawbacks (providing them with easy means of recruitment and indoctrination).
Also, please keep in mind that for fringe groups like these Reddit is not their primary form of communication. That is, the community leaders don't just PM each other on Reddit or communicate via posts and comments. If there is any kind of organized effort here to effect real world change, it is coordinated by private chat, often times with assistance of anonymizing networks like Tor. Reddit is where people come into this world, but it's only floor 1 of the a vast underground bunker (for the communities where these things exist). Take 4chan as another internet dumpster fire. Anonymous periodically puts on their own campaigns like pridefall which can absolutely create real world problems. But they don't communicate through memes. When stuff starts working on that level, they are coordinating these things via better channels and that's happening today. If 4chan was to disappear, I firmly believe that core group would still be in touch and looking to set up another dumpster fire on another social media site, be it a private Facebook group, a Twitter community, a Discord server, or something else. But I'd rather they spend their time chasing their tail trying to get back to having external visibility than to have them do it in the open.
Also, they already have the opportunity to act more covertly, and they do. I am getting pretty good at spotting bad actors, but I'm sure I miss more. Some are pretty clever in their online interactions. Kicking them off Reddit doesn't give them any better tools to do this. They already can and already are doing this today.
If you want further proof of this, consider why the members of the KKK wear hoods. Could they do what they do in the open? Maybe. But because they are your neighbors, your politicians, your police officers, and your friends, they need a level of anonymity. Take that away and you make it harder for them to do what they do. Reddit provides the means for that open yet semi-anonymous MO. Taking it away doesn't mean they change their beliefs, but it makes it harder for them to do what they do.