First off: we neeed some kind of coverage against steelix on the main set. It's clear that a Spinner that loses to, and gives a completely free switch to, all the SR users in the tier defeats the purpose of being a spinner in the first place. I feel that X-Scissor is a more expendable coverage, and you can instead opt for Earthquake (hits Camerupt) or Low Kick (hits fatties, and Steelix even harder)
252+ Atk Armaldo Low Kick (120 BP) vs. 252 HP / 0 Def Steelix: 152-180 (42.9 - 50.8%) -- before Mega Evolution
252+ Atk Armaldo Low Kick (120 BP) vs. 252 HP / 0 Def Mega Steelix: 134-158 (37.8 - 44.6%) -- guaranteed 3HKO
This is great damage to a big metagame threat that would otherwise get completely free turns to switch in and take potshots.
Also I know people are gonna laugh but I honestly think AV isn't that bad of an item choice on the main set if it is going to be 3 attacks + rapid spin. Yes, SD set doesn't want AV, and neither does SR sets. But if you have 3 attacks + rapid spin, with max attack Adamant slow bulky attacker, you don't want any of the damage boosting items. LO, CB, Plate is bad in these types of sets, so the situation here is that you're kind of forced into lefties even though it's not particularly synergetic to the set. AV lets you tank attacks that you shouldn't, allowing you to get a hard hit or spin off. Armaldo is bulky, but 6% isn't always something that will come relevant. Unless the damage calcs against the more relevant physical threats turn out that lefties actually turns 2hkos into 3hkos, lefties isn't any better than AV.
Of course losing the option to SR sucks, so if SR + Spin is the main set then sure, Leftovers it is, but if it isn't (and it's hard to get both Spin + SR as Armaldo) then AV isn't actually that terrible. It's not like SD is an integral part of Armaldo's success, so only thing you lose out is SR.