BizTalk Question:

Reasons for message immutable: A received message might have
multiple subscribers (orchestration, sendports etc) means each
subscriber of a particular message references the same, single
copy of that message. So, it make sense the message is not
modified by any single service like Orchestration or
pipelines.

2. Since single copy is referenced by multiple subscribers, it
minimize storage. ( A ref counter is maintained for each
message and messages with ref count of 0 are periodically
removed.)

3. The concept of messages being immutable also allows for
detailed tracking of message state as messages flow through
the system.