In a world in which all SOAP messages have <wsa:Action> header blocks, why do my Body elements need XML namespace qualification?
First order answer; I’d say simply because an intermediary that doesn’t know the value of the Action header might want to look at the payload.
Second order answer; I think it’s a fairly small & uninteresting world where it would it make sense to have Action headers within the SOAP envelope. In order for SOAP to make proper use of application protocols (well, transfer protocols at least), SOAP headers should be constrained to containing representation data and metadata, as that’s the data that remains constant between hops over different application protocols. For example, through a HTTP to SMTP bridge, a SOAP envelope should remain constant. “Action” is message metadata, and therefore does not necessarily remain constant between different protocols; it’s hop-by-hop.
Where it would make sense to put the Action header (and other non-representation headers) in the SOAP envelope, is when SOAP is used with transport protocols. But I don’t think there’s much value to that, at least in the short/medium term; the value of reusing established application protocols is too great.