This would make our file inclusion syntax, which is an atrocity that should not have been allowed to exist, even more complex. There is no way to tell whether a string is a parameter or the description unless you know what exactly the valid parameters are, so changes to them would force changes to every tool that tries to parse wikicode in a non-MediaWiki context.

Granted, [[File:Foo.png|no-viewer]] looks nicer in an article text than <span class="no-viewer">[[File:Foo.png]]</span>, but are there many use cases for that? Most of the problems I have seen are with heavily templated images like position maps. A few people complain about small images in general, but if that is really a problem, we should just filter on size.