It has several flaws, one of them being failure to check that session.save_path value is a default one for a given OS (or in other way restrict potential adverse effects of changing permissions).
Just fix your /tmp permissions. It restricts itself to running only once, so don't worry.
I can confirm this issue. We have seen /tmp permissons changed on all Plesk 10.4 machines after MU #41 was installed. Again a wonderful example of the lack of testing at Parallels.
Worth noting that we use /tmp as PHP's session.save_path instead of the default value.