The basic reason I'm high on Bender is this. Kelly Olynyk has managed to be a useful NBA player, and one of our better offensive players overall, despite his woeful lack of athleticism and his T-rex arms. That's because 7 footers who can make 3's and make good passes+decisions with the ball in their hands are automatically good offensive players. Even if they don't actually score a ton of points, they greatly improve the flow of the offense and they open driving lanes for your guards + wings. Bender can do that on offense, but he is also 100x more athletic than Olynyk, giving him the ability to blow by slower defenders, attack closeouts, and most importantly make plays in transition.
Meanwhile, Olynyk manages to be a passable defender by avoiding mistakes on that end, but Bender has the potential to be a game changer on defense. Paired with guards + wings that can switch onto bigs like Smart and Crowder, he enables you to "switch everything," an incredibly powerful strategy as demonstrated by OKC vs the Warriors. Normally, switching comes at a cost: you give up a mismatch, then you have to help on the mismatch, then everyone has to rotate and it's easy for the weak links in your defense to get exposed. But with Bender, you have the potential to switch without giving up a mismatch. IMO, this skill is more important than traditional rim protection, as it has become too easy to neutralize rim protectors by forcing them to guard shooters out on the perimeter.
Bender has a high floor, as he has all the key role player skills and it is difficult to see him failing to become a useful NBA player. He also has a high ceiling, as he is 18 years old with excellent tools + a great package of skills for a modern NBA 4.