A: "Ummm, i can't say for what is the reason"
C: "Ya, i think we dont need to delete that, correct?"
A: "Errr,  i dont think so"

This dialog between an IT auditor and his client is a lesson to us to understand more about user account. Here's the reason, (i wish, one day i could confidently answer this). So my brother, lets check this first before you talk: UNIX Security Checklist v2.0

And you can help your client decide where user account information is stored. Because IBM-AIX also suggest to remove unnecessary default user accounts.

To get more understanding check out the discussion at default users setup during install or again at faqs.org