No, because multiple jobs being run by the same end-user could share data files on the machine, in which case they needed to share the same uid. (Or alternatively we could have used the extra-gid trick to give shared group access to files, but that would have involved more on-disk state and hence be harder to change, versus the job tracking which was more ephemeral.) It's been a while now, but I have a hazy memory that in the case where a job was the only one with that uid running on a particular machine, we could make use of that and avoid needing to check the extra groups.