Why do Veigars go supp?

DOnt get it, Im seeing Veigars supp from time to time, now I got one myself. He was a lv 7 Veigar! Whats the point of this? The way I get it, Veigars strength mainly lies in his AP stacking, which can go to ridiculous levels. But supp veigars cant kill minions (of course), so their Q wont stack AP, so their AP wont go up to eleven as it should... The very thing his kit is built around goes away. Isnt Lux better as a heavy AP caster // punishing CC supp? Her shield is almost built to make her a viable supp. Or am I missing something?
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