About one time out of five when I update my system (~amd64) (emerge -uavDN world --with-bdeps=y), I get broken dependencies that need to be repaired with revdep-rebuild.
It seems like portage should be able to figure out if a library is being upgraded that is going to break some app's dependency on it, meaning that revdep-rebuild should really be unnecessary. As it is, revdep-rebuild takes a long time to run, checking absolutely every application installed. If it's impossible to tell a priori that a library version dependency has been broken, then at the very least, revdep-rebuild can be fed a list of apps that depend on every library that's just been upgraded and check only those apps.
There are similar problems that have to be repaired periodically with lafilefixer. It seems to me that this should be done automatically for any library that has just been upgraded.
.la files -> bug 271129
and portage 2.2 already has preserved-libs feature.
*** This bug has been marked as a duplicate of bug 271129 ***