I don't really see that as a bad documentation point, it's just a matter of policy. Offhand, I can think of very few sites that maintain a separate release version number in the URL. What if someone bookmarked or hotlinked something under there? I realize these are internal images for the site, but it's still just something that's not usually done.
That is, the way Seam is set up by default seems to be the more usual, less prone to unexpected behaviours method. I certainly wouldn't advocate changing the default to require using the /res<version>/ directory layout. That'd just be one more potential pitfall for people new to the framework. As it is, it'll work fine without that particular layout, and if the designer wants to go with a system as you're suggesting, they're certainly free to.
In summary, I think it's just you. :-)