Are there useless Gods?

Not in regards to Ares' passive, he only gets that extra power from completed aura items. Gauntlet of Thebes counts as one when it's evolved, so that can also be a good pickup depending upon where you play him.

I think some like Talisman and Tyrannical might. Easiest way to see is just check the aura items tab in game.

Oh wow that is good!

No, I'll give you 2 examples to show the difference between a passive and an aura.

Void Stone has a passive aura of 10% reduced magical protections in an area. This is an aura that affects all enemies near to you.

Stone of Binding has a passive that when you hit an enemy with crowd control it reduces their protections by 10.

Normal passives just effect you, or they effect enemies when you attack them. Auras effect enemies by just being near to you.

To answer your question in the title: No every got has its purposes. The only reason to become useless is getting nerfed in the ground even harder than Loki or falling far behind in the game (although this has a duration usually). In some cases, team comp also influences this.

For Ares he is very reliant on staying ahead. His early game pressure helps him a lot with it (good use of shackles can easily be half someone's hp on lvl 1). The problem is that he loses this pressure if you fall behind (and as the game goes on). In this case, he transitions more into an ult bot (and aura bot). if you don't manage to adapt and keep playing this high pressure you will feel the gap closing quickly.

In this case, you will start to take more of an initiator role. Trying to initiate with your ult and then try to keep enemies in place with the cripple from shackles and providing port with your 2 aura (and the effect of item effects)