I've been a Siaki Ika downer--on record as saying I wouldn't have taken him there. So, here's how I'd respond to my own argument with the optimist's take:
Siaki Ika will be a force in the middle who requires two men to block. He's an immovable object in the run game, and defense in the NFL often boils down to a numbers game.
If you can defend the run with fewer players, that allows you to be better against the pass. If you can keep two safeties back instead of bringing one down into the box, that gives you an advantage against what really matters in the modern NFL--the passing game.
So yeah, OOTRATQ, you might be right that Ika won't ever be able to rush the passer, but your vision is too myopic. You're missing the bigger picture. His strength in the run game allows our entire defense to be better against the pass.