Rubio is about the worst starting PG you can have and hope to be a playoff contender, IMO. He's a great playmaker but terrible shooter. He can score well enough when he has to, driving to the basket, but that, the occasional 3, and 3-4 FT a game is what will get him to his 12-15 points each night. He really only scores to keep defenses honest. If he didn't attack occasionally then they'd play off him entirely and keep him from getting others going, which he's really good.
He's not a Nash or CP3 level playmaker but the reason he's not is because play making is virtually all he does so it's easier for defenses to key on that than it is for Nash or CP3 because Rubio is never going to score 30+ and help his team win by scoring.
He's good defensively. He gambles for a lot of steals but usually he's smart when he does it and sees the open court right away so he'd get the turnover and a basket. He was slowing down when he was here but he was above average, for sure. He'd have trouble with speedy PG's but who doesn't struggle guarding them. He had good sized as well and wouldn't be abused physically by other PG's.
Im probably making him sound better than he is but after the awful PG's we had between Bledsoe demanding a trade and landing Ricky, he seemed like a godsend. We were running a bunch of G-Leaguers who would literally lose their spot in the NBA as a whole after we'd replace them with a slightly better G-Leaguer.
Overall I think he'd be the best backup PG in the league if that's the role he takes on and he'd definitely help a team contend in that role but as a starter he's not going to lead a team to a title or a conference finals. His best days are behind him at this point and he's really only a stop gap starter for a year or two but his team will be looking to upgrade that position sooner than later. Sort of like Crowder as the Suns PF. He's not flashy and won't wow anyone but he's adequate.
OKC did the trade because the CP3 trade because Chris wanted to come here and they were rebuilding. It made no sense to keep him. They got Oubre, Rubio, Ty Jerome, and our 1st this year. They flipped Rubio and Oubre to separate teams for 1st round picks and Jerome was just 1 year into his career. So they ended up with a young prospect and 3 total 1st rounders, which is pretty damn good for what people viewed as the worst contract in the league the previous year when they acquired him.