Since you can't fix broken libraries from binary packages, revdep-rebuild introduced having the ebuild maintainers placing a file in /etc/revdep-rebuild which would set SEARCH_DIRS_MASK to the directories where the binary package was installed. It would be nice if the preserved-libs feature also honored those files to prevent trying to repeatedly rebuild binary packages. The usage of these files is described in the revdep-rebuild man page under the configuration section.
Is there anything that SEARCH_DIRS_MASK solves that can't be solved by using patchelf, as described in bug #402081, comment #6?
SEARCH_DIRS_MASK is useful for revdep-rebuild, since it searches for unsatisfied/broken dynamic links. On the other hand, preserve-libs tries to prevent satisfied dynamic links from becoming unsatisfied/broken. When binary packages trigger preserve-libs, it typically falls into two categories:
1) Cases where scanelf --set-rpath '$ORIGIN' can be used (see bug #265372 and bug #400979).
2) Missing dependencies (like a legacy libng or openssl slot).
If those are the only cases where binary packages trigger preserve-libs, then preserve-libs doesn't need to use SEARCH_DIRS_MASK.