I own the Huskk leather wallet case and love it. The phone is slippery without a case and I love combining my wallet with my phone. But, there's no protection from falls/drops, so you still have to be careful with the phone.
I got myself quite a sturdy case for my 6+ mainly for protection on the corners of the phone in case of drops, it also allows me to put the phone face down on a surface without risk of screen damage. The 6+ is notoriously slippery and the rubber feel to the case makes it feels less likely that I will drop it
I guess the slippery nature should be the best idea for having to use a case else you stand the risk of loosing the screen of the phone. Meanwhile, casing tends to make the phone a bit larger which makes it quite large when you handle at a point it does not feel like a phone any more. That's for me thou.