The main reason that the controller needs an ip interface in the guest VLAN is that the controller needs to redirect the guest traffic to an ip address to bring up the captive portal page. Guests normally do not have access to the management VLAN of the controller to see the page, so what you should do is - create an ip interface on the controller for the guest vlan, and then use the "ip cp-redirect address" command to point the guest traffic to that ip address:
config t
interface vlan 300
ip address 192.168.1.5 255.255.255.0
exit
ip cp-redirect address 192.168.1.5