They should be able to see the contents of the forum, yes. Whether or not their comments will be regarded equally by the community is up to the community, just like it is in every discussion everywhere. If the come here to cause trouble, they are removed. If they come here to participate, then we want them to be able to read the discussions here, understand the tone and the community that we have, and then join in.
I know this goes against the design of your software, which (I preusme) reflects your idea of community building. But long before BBS, the Boing Boing discussion area was a place where anyone could read and comment on discussions. Discourse has allowed us, for the first time, to have the community create and foster some of those discussions, and to build relationships as a result, and I’m glad that’s the case. But not in a way that segments that discussion, and as a result, the members of that community. There are enough walled gardens out there to do that.