Having the full X509 distinguished name as primary user ID and a slightly modified version for the on-disk home directory made sense back when migrid started and relied on user certificates, but with the introduction of other authentication methods etc. it has become ever more clear that it introduces artificial limitations and complicates quite a few things in the stack.
We're looking into reworking the handling of IDs and home directories to sever that rigid binding.
Changing it now of course is a bit tricky because quite a few places in the code and Apache setup more or less clearly make assumptions about it being so. Still it makes a lot of sense to proceed to allow things like easy adjustment of user affiliation fields without e.g. a costly full home directory renaming.