Channel response serialization

I'm curious about the justification for the requirement that responses to
queries on a channel be in the same order in which the queries were issued.
I understand that it has been stated that this simplifies the protocol
implementation. Does this mean that it simplifies the protocol or the
libraries that implement it? If its just the libraries, couldn't the
serialization requirement be made optional (i.e. part of the session
negotiation)?

I ask because it seems that this excludes a large class of applications from
using BEEP, so that those of us who need truly asynchronous messaging are
left with the same problem that BEEP was intended to solve. Namely: HTTP
doesn't quite fit and SMTP doesn't quite fit.

Now BEEP doesn't quite fit either :-(