The usual reason to discard the 8BITMIME ESMTP keyword is to make sure things that depend on 7-bit clean message bodies work properly. The most obvious example is DomainKeys/DKIM signing. If your message is signed as an 8-bit message and later downgraded to 7-bit by a downstream hop, signatures will break as content is converted to Quoted-Printable (or less likely, Base64).
Forcing the upstream hop to convert to Quoted-Printable before delivering to your Postfix server (i.e., not advertising 8BITMIME in the EHLO response) is the easiest way to get a clean message for your content scanning/signing stuff. At a guess, if this was in your distribution's default main.cf, the packager wanted to make sure you had a lowest-common-denominator setup with no nasty surprises when you add features later on.
In general, trying to get Postfix not to log stuff is more trouble than it's worth. Better to post-process the logs via scripts (or in your syslog daemon if it has the capability).