As DLR says Finkelstein is not just using these outlets, he is being used by them. The same as those who presented programs on RT were being used by the Russian state. To argue that any platform is ok so long as it gets your message across is ludicrously shortsighted. Giving cover to the harm that RT, Brand, GBNews or whoever is one factor that needs to be balanced against any extra coverage one obtains.
But there isn't any real harm. He's giving them prestige? Oh come on. He's giving them views, and in that way he's helping them out personally, but not politically. That's pretty sour in the case of Brand, but only because of the allegations.
Engaging with any media is a tactical and strategic question and I don't have a fundamental problem with socialists engaging with liberal or conservative media outlets. But one has to go into it with one's eye's open, and with a need to ask questions like who is the audience you are trying to get your message over to? How is the media going to use you for their interests? Can this be used by political opponents for capital?
I've seen Finkelstein on some of these shows. He doesn't compromise his message in the slightest although that may not be your point. The question is, what is your point? How is this being used by these outlets?
In the case of Brand you have an audience of hardcore tinfoil hat libertarian shits, not some mainstream, or even smallish, media outlet that a significant proportion of the general public watch, or are at least aware of. This is not someone going on GBNews or being interviewed by Piers Morgan, to try are engage with a reasonably sized number of conservative (leaning) viewers, it's a very nasty rabbit hole full of shits that are publicising a series of harmful choices and actions. Shunning conspiracaloon YouTube channels, particularly ones run by abusers, is a pretty obvious political decision.
This is the crux I think. I think that you can argue that Brand's audience have the power to disrupt through the poison of their conspiracy theories and that they shouldn't in any way be encouraged. But this is still so hand wavey - they're being encouraged anyway by the fact that they are Brand's audience. What are the actual consequences of appearing on his show? What is the real harm?
Socialists should be prepared to speak to EVERYBODY. That's a long standing, well established principle. "You can't talk to these people because they're really bad?" Where does that idea even come from? What's the tradition that says that?