In fact, when we have big network with many VLANs or two DHCP in one VLAN - we may have situation when some important locations are INACTIVE, or client can be putted in wrong location (which is worse).
1. two BRAS servers with different DHCP subnets in one client VLAN (for failure-resistance purpose). If AP receive IP address from one BRAS, second location will become INACTIVE for this AP. If first BRAS goes down, AP will know that only after reboot and clients will have no service untill AP check locations.
2. In ISP network with L2 VPN in core (VPLS network) with many different VLANs as last mile which are connected to L2 domain in VPLS and big IP subnet on BRAS it is possible that in one location VLAN will lead to public zone BRAS interface, and on other location the same VLAN ID will lead to private BRAS interface - with different restrictions and policies. When we use open SSID with one name for entire network we can't specify roles and location groups to prevent this situation.
We see two ways to solve this issue - allow to create many SSID with the same and some uniq internal number and bind different Roles for this SSID, OR - allow to put clients directly to VLAN without subnet checking. Second option looks better for us