Append only has some nice properties. It's a lot easier to sync systems, because you only need 1 number for each feed, the latest sequence, this makes things like EBT really efficient. You could do something like a set sync instead, Aljoscha has worked on that. In any case, deleting data in a distributed system is a hard problem. If you do separate the content from the log as bamboo and buttwoo does, then you still get the nice properties of an append only system, without having to store the content potentially.
@gwil btw I build this module that could be interesting to you for set replication: https://github.com/arj03/ecmh-js