It has a place in some comps. If you expect a fight to drag on, and your adc is the deciding factor for it, suicide karthus can transition into midrange karthus, which is inbetween your front line and your backline to support both. You want to drag aggro from the enemy frontline so your adc has more free reign. Using a GA allows you to add an additional 3-ish seconds of e defile, which can be pretty useful for helping an adc. That being said, it's far from optimal, considering standard karthus build is usually archangels, roa, deathcap, void, zhonya's, sorc boots, replacing zhonya's or roa for cdr or rylai's respectively.