Yeah I was totally against the idea of getting Thomas .... until the Lakers got Gasol and the Suns got Shaq. Now that the West has gotten bigger, Thomas makes more sense.
I do agree that this trade doesn't help against many teams. Who besides the Suns and the Lakers does Thomas play against? He'd get destroyed by Dirk. He can't guard David West. He can't guard Okur. He doesn't help against Houston, Denver, Portland or Golden State.
In the East, he doesn't help against the Celtics because Garnett would just take him away from the basket. Ditto with Rasheed and Detroit.
It appears as if the Spurs now see the Suns and Lakers as their two biggest threats. That's the only reason you make this trade. Against those two teams, Thomas will be useful. Against everyone else? I'd rather have Oberto out there.