You need the Dark Below DLC when the weekly Heroic and Nightfall are Omnigal. The Omnigal strike will replace one of the usual ones [u]once a month[/u]. Its not known yet whether the coming HoW strike will rotate with Omnigal so we still get one DLC strike each month, alternately. Or if the HoW strike will replace another strike once a month also, leaving those without DLC even further screwed. Time will tell.