Love the iPhone Air, but I hate the polished sides and putting it in a case kind of ruins it for me, even Apple's bumper case, which should have been more of a super thin Air bumper case, not a traditional one. The bumper case will still add bulk to it which kind of takes away from its selling point, the thinness of it all.
As for the polished metal sides, I think aesthetically it would've looked better (and thinner) with body color or slightly contrasting matte sides. A thin slab of titanium. However, I am wondering if they went with the polished sides for better grip as matte sides would make a very thin phone more slippery and harder to hold.
Anyone who buys an iPhone Air just to slap a case on it is kind of defeating its purpose, Apple probably knows this and may be estimating that the Air may be the iPhone that sees less case usage, and the polished sides may have been their attempt to make it less slippery to hold and to add more bling to showcase the new design for other potential customers to see.
However, I am sure most people will still put the iPhone Air in a case and still enjoy its thinness compared to the larger iPhones in cases as it will still be relatively thinner by comparison, it will just lose that 'wow' thin-factor at first glance. Even Apple's bumper case wipes out the overall thinness visually. I wish they would've made a bumper that was also super thin with just a light edge-raise all around with an accompanying and matching little silicon camera ring.
Speaking of bumper cases, I also wish Apple would've released them for the entire iPhone lineup.