Kiln Harmony is a little bit different than that. We are aiming that you can push a Git repo to Kiln, clone out an Hg repo, push that back to Kiln in an empty repo, and get back
exactly the same Git repo you started with. This means that both format types are completely equal--and, more importantly, they're both
canonical.Plastic and Perforce are solving a very important, but fundamentally different, problem: how can I use Git to interface with my proprietary SCM? These problems tend to evolve solutions like git-svn or hgsubversion, where either you're keeping the initial repository around as a base (I believe Plastic does this), or you simply don't even care about it at all (e.g., in Perforce, its repos are assembled from arbitrary slices of the main source tree, so the same commit may have different files entirely from the one your neighbor is using, even though both repos include the same chunks of history).
Those are also valuable tools, and we did consider making what would amount to a Git front-end for Mercurial. But we really wanted both systems to be peers, both canonical, and that mandated a totally different set of design decisions and restrictions.
It's not that what Plastic and Perforce have aren't cool. They're just fundamentally different.