When I was new on the 320 I regularly used MCT when
instructed to expedite climb - then all hell broke loose in
the flt dept when they found that me and my batch (all
ex Boeing) were doing it. We never ever got a definitive
reason as to why, in pilotage terms, we can't damn well
use it to expedite climb. But its their sand castle so we
just shutup and did what we were told.
Only once did I use MCT in cruise when I was dumb
and silly enough to leave the cockpit for an expended
urgent dunny visit with a 200hr wonder at the helm. We
were in an area of CBs but the hdg I set beforehand
was smooth and told Spanky not to change hdg or alt
unless we lost pressurisation or an engine. We were at
REC MAX - 300ft and about 1000ft over OPT due to
enroute ATC constraints further down the track which
would've duckshoved us to FL270 for the remaining 4
hours.
Cutting a long story short - just before I got back the
space cadet had changed hdg and steered us straight
into a bloody cell where the TAT went up by a large
10* and max CRZ power wasn't enough to keep the
speed from dropping off - so I went to MCT. Young
eager-beaver said he saw a break to get closer to track
but failed to scan beyond 10nm and took us smack in
to a blind alley.