Rationale for the "Never commit to the right side of a Pull line" rule

Hi,

Could someone please point me to / give me the rationale for the "Never 
commit to the right side of a Pull line" rule ?

I found the rule in the second Note of the git-fetch man-page (eg 
http://www.kernel.org/pub/software/scm/git/docs/git-fetch.html).

It's been taken over in _bold letters_ by X.Org/freedesktop.org in their 
"UsingGit" document (http://freedesktop.org/wiki/UsingGit).

In both places though, I don't see any explanation, but rather a 
commandment ;-) . Am I missing the ovious ?

My candid thoughts: to me the practice recommended here seems 
subjectively "cleaner" indeed, but is it objectively better or even 
essential? Why?

TIA
Jérôme