They're not begging, so they absolutely can afford to be choosers. lol, you think the Celtics are the only guys they can trade Ibaka to? They don't have to (and they wouldn't) settle on Marcus Smart.
1) Check your facts. He shot 31% from 3.
2) That's bad. Still significantly better than Smart's 25%.
3) He can pass.
4) He's the fifth option... all they need from him is to hit shots. They don't need him to do much else with the ball.
We don't know what the trade market is, but they won't likely be getting a quality shooter who is an upgrade at that position for a 1 year rental of Ibaka, unless the incoming guy is a rental too, or much older, etc.
EDIT: We sort of know the market, since 2 years of Thad Young went for #20.
Roberson is career 27% on one attempt per game. Definition of wide open shots.
Smart is career 30% shooter on four attempts per game. That's actually a big difference, and Smart had a career low last year. No reason to believe he stays there.
No, he can't pass. He averages 1.5 assists per 36 minutes. It's not a hidden skill, if he had any passing skills at SG he'd dish out more than 1 assist per game playing with Durant and Westbrook.
As for the 5th option... yeah, because he's that bad offensively. The fact that OKC can afford that has nothing to do with Smart vs Roberson.