Actually, it occurs to me that my previous comment makes the case that SI should have a single logical document, kept up to date, maintaining its current case. It doesn't argue that it should be a wiki. One alternative would be to keep a book in revision control - there are doubtless others, but let me discuss this one.

Pros:

• A book has a prescribed reading order; it may be easier to take in the content if you can begin at the beginning and work forwards. This is a huge advantage - I'd upload it to the Kindles of all my friends who have given me permission.
• The book would be written in LaTeX, so it would be easier to convert parts of it to academic papers. MediaWiki format is the most awful unparseable dog's breakfast; it seems a shame to use it to create content of lasting value.
• Real revision control is utterly wonderful (eg hg, git) - what MediaWiki provides absolutely pales in comparison.
• Real revision control makes it easier for outsiders to contribute without special permissions - they just send you patches, or invite you to pull from them.

Cons:

• Mediawiki is easier to use
• People are used to wikis
• Wikis more naturally invite contribution
• Wikis don't need you to install or compile anything
• Much of the content is more wiki-like than book-like - it's not a core part of what SI are discussing but an aside about the work of others, and in a book it would probably go in an appendix.