alt Hacker NewsB/c, by design, you want the archived stream of events to include everything.
e.g. a lot of these systems have a "replay" node that can be used by components that just restarted. You want the replay to include ALL of the messages seen so you can rebuild the state at any given point.
(There are, of course, tradeoffs to this so I'm just commenting on the "single sequencer" design philosophy)
by definition: an exchange doesn't need any reference to outside market data
even for systems built on a sequencer which do (e.g. an OMS), the volume is too large
the usual strategy is for processes which require it, is to sample it and them stamp it on commands
which maintains the invariants
(my background: I have been a developer on one of Mike Blum's original sequencers)