I get what you're saying. However, I'd probably apply the kernel rule of "when maintaining the kernel, do not do something which breaks user programs/applications". Yes, this isn't the kernel, but it's comparable in being a core function that heavily affects userland stuff.
Sometimes the ole way o' logg out is just insecure. And there is no way to conjure up a new backward compatible and secure way. cgroups work well, especially because they are not opt-in. That means programs daemonizing either has to set themselves up as a system service or start a new logind scope (or PAM session, etc. which translates to escaping the cgroup, which requires user approval to remain secure).
