currently there is no 0.17 ebuild in the tree so only 9999 should depend on them. see http://btrfs.wiki.kernel.org/index.php/Getting_started
This is valid, but I'm not going to touch these ebuilds now, since they will soon be obsolete anyway - btrfs is in the kernel itself as of 2.6.29... Also, the default gentoo kernel config already includes these options.
Opening because we now have 0.17
Ebuild contributed for 0.17, and 9999 changed to include this check.