So listen, not that I'm really into defending the Oilers, but I kind of feel everyone is being a little unfair...but then again it's the Oilers and nobody deserves to be fair to them.
However, here is the Hero Chart for Klefbom last year.
Enoch Root is correct in saying he still makes major mistakes, but that's what you would EXPECT for a rookie defenseman. Brodie was not much better in his first full season. What you want to see is growth and learning from your mistakes. In that sense, everyone is correct in saying that Chiarelli could have easily waited to see how he performed this year (especially under McLellan's tutelage) before doing this deal.
The other thing is that by all accounts Justin Schultz played much better when paired with Klefbom. He'll never be confused for a defensive stalwart, but if they can get Schultz playing passable defense at the NHL level that's a win, especially when you consider his offensive upside (think Mike Green).
Also, one last thought: While the cap has stagnated this year, it isn't out of the realm of possibilities for it to still go up over time. Consider that many regular 3rd pairing d-men are now regularly being offered salaries in the $3 million range, it's not ridiculous to think that they could make somewhere around $4 million in the future as a standard. In that line of thinking, EVEN IF Klefbom were no more than a 3rd pairing guy, it still wouldn't be a terrible contract down the road. I think he'll likely be a top 4 guy, especially if he's coached well.