Except that mod_perl is still a single global environment for all Perl programs such that one customer's global changes could overwrite another customer's global changes.
That's true for server config changes, but not for anything you can do after server startup (i.e. without root). When you set MaxRequestsPerChild to 1, everything is gone at the end of each request.
I don't see any need for mod_perl to compete in the hosting market when FastCGI is already ubiquitous, and has this same capability. (The Ruby hype was good for something after all!) I think these guys should have taken a closer look at FastCGI before they decided that cheap hosting was a problem.