Agreed. They should only be low latency within a dense community, like say at a rally or in an urban neighborhood and even then, latency on the order of seconds is fine. As it is almost always more important to o communicate with those socially proximate to you than those further away.

I think the level of indirection and timelyness can be used as a filter, in that it ought to be modeled after the sneaker nets and human crowds/socialization. For example individual clients can ignore messages more than a configurable signed time/distance and configure their clients to only repeat at a certain rate. That way the network is robust to spam, because it relies on the movement and consensus of people to move messages long range.