I guessed that it was probably pretty core. Maybe an enhancement request to MediaWiki pointing out the ambiguity in this buttons functionality then would be good. I reckon it should say [[<link title>]] rather than [[link title]]. Because as it stands it is not consistent with what the external link button offers up. which is [http://www.example.com link title] So for consistency the external link one could actually be [<http://www.example.com> <link title>]
or maybe [http://www.example.com LinkTitle] and [[LinkTitle]] But perhaps this is a discussion to by had on the MediaWiki web site not this one.
I will shut up now.