What is the technical difference between using "w" as the name of the directory and "wiki"?
One benefit of installing Mediawiki into a directory different from the one it is (or appears to be, via mod_rewrite) served out of is that you can disallow search engines from indexing the actual installation directory. This will prevent edit pages from turning up in search results. I believe this is one of the reasons Wikipedia takes this approach.
The name of the installation directory doesn't matter, as long as it's different from the "virtual" directory you want the wiki pages served from. One of my sites uses a method similar to the one in the Mediawiki docs, with Mediawiki installed into /mw, while the content URLs begin with /wiki.
Using this approach, you can then add the following to /robots.txt:
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