Durability, or, Lack Thereof
We'd be fine with the slippery design if the iPhone 6 and iPhone 6 Plus were protected by impenetrable materials. But they aren't.
The all-aluminum design is succeptible to dings and cuts and scratches. We used the iPhone 6 without a case for several months. We dropped it a few times from pocket-level and the device immediately became dinged up and scratched.
The iPhone 6 also started to show general wear and tear much quicker than other flagship smartphones. It's a trade-off. You get a premium metal design but you're essentially forced into covering up that premium metal design with a case. Fortunately, the iPhone 6 and iPhone 6 Plus wear cases well.
Cases cost extra money and for many people (myself included), they're not ideal. We want to be able to show off the iPhone 6's design but we can't because using the iPhone 6 or iPhone 6 Plus without a case is a disaster waiting to happen.