Another pro is that the core code can always use more review. There are bugs, and sometimes they are surprisingly findable.

For instance Ovid also wrote use CGI or die;, which includes among the benefits of CGI.pm that hand-rolled solutions don't tend to test $ENV{CONTENT-LENGTH} to catch aborted downloads. Well it was by reading the source-code that I found that a hard-to-debug problem happened because CGI.pm didn't either on a regular POST. (Only on a multi-part post.)

And even if you don't find any bugs in reviewing a bunch of that code, you will get insight into ways that very smart people who you have never met think. This is never a bad thing (even if the state of the art has progressed).

For a possible, if anyone wants to read Code Reading: The Open Source Perspective and get back on how it is, I would be interesting. I would have read it, but I have been busy learning stuff (eg Oracle) for a new job...