I can't speak for anyone else but I did it out of habit. I tend toward Tesla Immortals, so the anti-tank Staff helps the squad in that set-up. I made the switch to Gauss, given they'll be in Rapid-fire but the other habit stuck in there. Not complaining as 24 anti-vehicle shots on my first opponent will help a lot.
But speaking of AP 3, how come none of us Necron went with 5 Destroyers or 5 Lords? I was thinking quantity over quality: hasn't failed me yet. Maybe I should have gone 15 Warriors.
I considered it strongly, but there were some sticking points.
1: Destroyers aren't that great, lets be real. I'll take 20 AP4 S5 shots over half that many AP3 shots any day, even if they are Preferred Enemy. They really gutted Destroyers in the new Codex.
2: You're paying for Jump Infantry, something that will never come into effect. Inefficient points usage.
3: Lords intrigued me but those are more CC based. A lord costs considerably more than an immortal and doesn't offer too much in return, especially on the anti-vehicle front.
4: Warriors are a bit TOO fragile. Going from 4+ to 3+ armor saves is a huge jump in survivability and you only lose 3-4 models to get it. Warriors exist to either tank hits as a huge mob of 20 with a res orb, or as 5-man squads for objective taking/contesting on later turns.
Overall Immortals were, in fact, the best choice. If we were allowed Pinning though, I'd have taken Deathmarks and made you all cry...