I'm quite ambivalent about this rule, often disable it, and would not mind removing it altogether.
I gather by "this rule" you mean the syntax-check rule. I agree completely. I doubt rms would agree to removing the original statement from standards.texi, since it's a user-visible convention. However, it's clearly intended to apply only to messages of the form PROGRAM:SRCFILE:LINENO:COLUMN: MESSAGE Diagnostic messages of other sorts, like: Input files contain messages in different encodings, %s and %s among others. certainly should be capitalized and have periods. That is indeed a sentence :). (Although, switching topics a bit, I wonder if users might like to have all such errors cast in the above format so that next-error would do something sensible with them ... anyway ...) k