Actually, the wp-webp plugin just adds WebPJS (http://webpjs.appspot.com/) to the site. This project has not been updated since December 2012 (see http://webpjs.appspot.com/Changelog.txt), so the wp-webp plugin doesn't have the most recent specifications.
Since last December,the libwebp version is 0.4.0 (see the download repository on the website) and now supports transparency and much faster decoding (I'm talking from seconds to milliseconds), and is of course less buggy (and it ends the choice to make between WebP v0.2.1 and v0.3.0 that was problematic before).
So now that even Google considers WebP stable enough to use it for Youtube previews, should WordPress supports it natively? I'd say it depends of what it the priority.
_If the priority is to make a much faster WordPress, then the current specs are just fine since WebP images. For example, I converted a 820 KB transparent PNG comic image already compressed (with TinyPNG, if you want to compare with my results) I posted on WordPress to a 222 KB WebP image. While WebP is currently only supported by Chrome/Chromium, Opera and Android Browser, that would be enough for significantly saving bandwidth and speeding webpages loading and display, which is very interesting for websites managers willing to save as much costs as possible (maybe even for WordPress.com for the same images of the website side only if some converter is developed).
_But if the policy is to support natively a much stabler version, then I think we need to wait before deciding if it's worth to implement WebP native support on WordPress. Or to bring it as an optional and experimental feature in Jetpack so developers could decide.
There, my 2 cents about this. :)