Cache-control: proxy-maxage as MUST for HTTP/1.1?

Jeff Mogul writes (privately) with respect to hit metering:

# ... the most important prerequisite is that the WG agree on our
# proposal for adding Cache-control: proxy-maxage as a MUST for
# HTTP/1.1.  Once we have this nailed down, a fair amount of the
# discussion in the hit-metering draft can be deleted and/or made more
# concrete.

So, what do you think about this? (Technically, avoid the "process"
Proposed/Draft etc issues for the moment.)

Larry