> What I do like about fix clients (not fix protocol) is the extra value they provide by keeping your sessions alive, tolerating hot restarts, logging in and out on schedule and managing multiple sessions.
I agree somewhat, but I think the protocol needed to come first. Can you honestly say that in its absence, multiple clients would be able to implement these behaviours in a uniform manner?
The fact that there's a concept of a "session" demarcated with LOGON/LOGOUT, sequence numbering, resend semantics, heartbeating - all this had to come from... somewhere.
Put another way: if you get 10 brokers developing their own REST/HTTP API's, what is the chance they'll normalise on a similar set of core concepts?