I guess one "downside" to the lib-based XML modules is that they seem to depend on something that you don't get from CPAN: James Clark's expat toolkit.

Of course, that's an easy thing to get and easy to install -- it took me less than 5 minutes to google it, download it, compile it (just "make"), and move it to /usr/local/expat -- and it's presumably easier on any Windows box, since there are precompiled libs in the package. So having XML::Parser depend on expat is not a problem, I think. (In fact, I'd rather have expat as the guts for an XML parser in Perl -- James Clark is God in this domain.)

For that matter, if people are going to be doing stuff with XML in general anyway, there's no good reason to avoid having "expat" available, just on general principles, whether you use Perl or other things on XML data.