In what kind of deck is Aether Vial good?
Obviously Aether Vial does best in a creature-heavy deck where many of the creatures have low CMC. Having to tick up a Vial to 3 or 4 means waiting 3 or 4 turns before it gives you any value.

In addition to the points you listed above, one advantage that you missed is that creatures put into play by Vial are not cast, and therefore sidestep counterspells and related effects.

This property also enables the Vial to break symmetry on effects that prevent players from casting spells, or affect the costs to cast spells: (e.g., Trinisphere, Standstill, Sphere of Resistance, Rule of Law).

Multiples are not completely redundant: in addition to being able to vial in multiple creatures per turn, you can hold the vials at different number of counters, for creatures with different CMC.

I would say the reason you don't see it more often in competitive play is that simply putting more creatures on the board isn't always that strong of an effect. Many decks with lots of cheap creatures are on an aggro plan, and so Vial is both tempo loss and card disadvantage. The positive benefits of Vial accrue over multiple turns, so it does the most work in a deck that wants somewhat longer games, and also has lots of cheap creatures, preferably with redundant synergy (e.g., Fish).