Why see if a CSS exists?
It adds more load to the webserver, as it has to handle a lot of
"404 Not Found" requests, and the webserver error logs start
filling up with all of the 404 error responses. In fact, if no
pub/css/local.css, pub/css/Group.css, and pub/css/Group.PageName.css
file exists, then each page request will generate three extra 404
entries in the webserver logs.
At any rate, people will see that PmWiki is generating 404 errors
in the web logs and think "Gee, that's not a very robust
Some people (especially people in the business of web design)
will have their browsers configured to report when 404 errors
occur in images and .css files, so it'll be less than ideal for them.
The page won't pass some validation tests (e.g., on w3.org)
that check to make sure the .css links are valid.
So, while some people might not care about any of this, there are
a lot of people who would consider these to be fatal shortcomings
of the PmWiki implementation and choose to go with something else