I think that it is defined internally in
MediaWiki (you don't see a
"summary" field when adding a new section to some talkpage, do you?),
so I think it would be counterintuitive to provide a new parameter
when "summary" will be just ignored.
Well I could go ahead and change
that internally, but MediaWiki 1.11 and
later auto-generate a summary like "New section: [[Foo#Bar|Bar]]". In
1.11 this broke terribly if the section title contained special
characters, but it works fine in 1.12 and later. For this reason I think
overriding the auto-generated summary wouldn't be of much use and could
be counterintuitive and/or confusing.
Roan Kattouw (Catrope)