Decided to see what the raw statistical benefit of this item really is on two of its most popular users. With 252+ SpD, it takes Scizor to the equivalent of a base 145, an effective 65-point jump in base stats, and it takes Tyranitar to the equivalent of a base 175, a 75-point jump. This is before taking Sand Stream into account, in Tyranitar's case. Comparing to their Mega Stone bonuses:
AV Scizor: no Atk, no Def, no SpA, +65 SpD, no Spe (total +65)
Mega Scizor: +20 Atk, +40 Def, +10 SpA, +20 SpD, +10 Spe (total +100)
AV Tyranitar: no Atk, no Def, no SpA, +75 SpD, no Spe (total +75)
Mega Tyranitar: +30 Atk, +40 Def, no SpA, +20 SpD, +10 Spe (total +100)
This should make it easier to quantify the merits one way or another. The Mega Stone gives an overall larger stat boost, but not by as big a margin as you might think, and it's not as focused, so AV can certainly be more useful in the right situation. On the other hand, it does retain the ability to use non-attacking moves, which can be particularly useful on a defensive set.
Also might be worth noting that without SpD investment, it only takes Scizor to the equivalent of a base 130 (50-point increase) and Tyranitar to the equivalent of a base 160 (60-point increase). Still substantial, but not quite as huge.