I think it would be a good idea to ship a default config file with MT that includes all settings (commented out, of course). Additionally every setting should have a concise explanation of what it does. That is what other software distribution do as well and it is extremely helpful for newcomers or if you need to add some setting and you forgot syntax/spelling..
Reported on Movable Type 5
I don't believe in that concept!
That would transform setting up a configuration directive into a nightmare as you would have to swim within a sea of items and descriptions. Something like a mt-config.cgi file measuring a few MBs :)
Instead, you could study about Configuration Directives at:
http://www.movabletype.org/documentation/appendices/config-directives/
You are absolutely right. That's why this should be more of a reference configuration file that is never to be used in production but for reference purposes, ideally grouped in some logical way.
The documentation for configuration directives is nice, but I don't see any hurts in providing a sample mt-config.cgi-full or so.
Imagine how some people would complain that the movable type installation got say 2-3 MBs bigger only to contain that file.
I really don't think this is feasible, considering that there is a specific section on the documentation for configuration directives at:
http://www.movabletype.org/documentation/appendices/config-directives/