I have encountered a minor downside. Occasionally ActiveState removes modules from PPM. The one time I needed one of those modules, another monk was able to give me directions to get a compatible copy.
ActiveState also ships with cpan, build tools are available via ppm, so there's nothing to stop you building modules direct from source. I prefer to use Strawberry Perl when I have to work on windows, since it comes with a sensible set of modules on top of core, and the development environment is complete, no need to download anything else.