It's all a matter a preference really. Whether you want to have an active-active vs active-standby. Yes, an active/active configuration requires more disks, because of the additional aggregate and raid groups, however both controllers are actively utilized.
Keep in mind that even if you want to assign all disk to a one of the heads/controller you can't, the standby head/controlle still needs to have a root volume, aggregate and thefore raid group in order to boot ONTAP.
http://www.netapp.com/us/system/pdf-reader.aspx?pdfuri=tcm:10-61088-16&m=tr-3450.pdf