Walked into new job with an active network. There are two comm closet on the same floor that each have a 4510. The 4510's just have a management VLAN being trunked to a 6509. Both switches are in transparent mode. Both 4510's have the same vlan 300 (client end) but different subnets. I was always taught to have unique VLANS for each subnet. Not having any noticable network problems, just found this strange procedures.
You are correct for nothing more than it keeps it from getting confusing . Also you could never tie those switches together with a trunk if they have different subnets with the same vlan.