Strictly speaking, this doesn't block multiple communities. A single relay can host an arbitrary number of communities. Our current implementation will accept and host any community that you announce to it.
In the future, I think we'll build configuration options so that relays only host a pool of "known/trusted" communities, but (like so many things) we haven't needed that yet.
However, having this would enable a level of resilience within our infrastructure that I find appealing, so I definitely want this.