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There is a very definite reason not to do this in the general case.
The people who have set up CPAN mirrors are donating their bandwidth and storage. If we started uploading binaries (be it PPM or .par's) of all CPAN modules for all versions of perl and for several OS's, the size of the CPAN archive would explode. That might or rather will be considered abuse.
A further issue is that the distribution of binaries from untrusted sources is a major security issue. Suppose anybody could upload a binary for any module. Madness!
Now, laying these points aside, let me tell you that I am regularily uploading binaries to CPAN. Yes, you read that right. I'm violating my own advice here.
Reason for this lies in the nature of those binaries: They're binary builds of PAR (now PAR::Packer) for win32 only. Mainly, this is because PAR has itself traits of a package manager and providing a binary can mean the user does not need to do fancy bootstrapping to get it to work. This has been relaxed now that PAR was split into two distributions, however. The other reason is that Win32 is one of the major user platforms for PAR and doesn't always come with a C compiler. Furthermore, the security issue is sort of minimized by that those packages are always by the same CPAN user as the release manager for PAR itself.
That being said: Why upload PPM's to CPAN which can only reasonably be used with a single specific distribution of perl? (ActivePerl)