"Bob Gardner" wrote in
news:5cN2c.197384$jk2.708530@attbi_s53:

With reference to using Mode C as a defacto report....

5-5-6 Exceptions
{New-2003-15 a. revised August 7, 2003}
a. Do not use Mode C to effect vertical separation with an
aircraft on a
cruise clearance, contact approach, or as specified in
paragraph 5-15-4, System Requirements, subparagraph e and
f.

Maybe not right on point, but a suggestion that
controller's do not necessarily buy a Mode C readout all
the time.

They may buy the Mode C readout, but they can't trust the
aircraft to stay at the altitude they're reading. A cruise
clearance permits descending and climbing at pilot's discretion.
We use it all the time in the Gulf of Mexico, for instrument
approaches where we can't talk to center once we start a
descent, or often at altitude. We don't often climb back up,
but we might if we lose comm with both center and our company
flight following, and have to climb to regain contact. On a
cruise clearance, the aircraft may either climb or descend,
without the controller having any control over it, thus he can't
rely on the altitude readout to separate traffic.

--
Regards,

Stan