I have had a long line of Apple laptops going back to 2009. I had a white MB, 2011 MBA 11" & 13", 2011 15" MBP, and now 2016 13" MBP.
My personal opinion a sleeve is added protection when traveling in a bag. A cheap insurance.
I used many hard cases but never really satisfied. I usually remove the top portion after a short period because of the added weight on the top and it takes away from the beauty of the laptop. but I do like the added protection on the bottom covering the rubber feet. I wish they only sold bottom portions.
I used a iCarbon skin on a MBA and it looked nice but not much protection. Again I like the feet covered so when I am out in the field putting the laptop on a unclean surface I feel better it is contacting the plastic hard case instead of the actual laptop
As I type this now I have a few sleeves from the past I use when putting it in a bag. I bought a iBlason
https://www.amazon.com/Macbook-i-Blason-Rubberized-13-inch-Release/dp/B01MPYI8SA/ref=sr_1_6?
ie=UTF8&qid=1484108127&sr=8-6&keywords=macbook+pro+2016+13+inch+case
hard cover but I just took off the top and am only using the bottom. I recently pitched a few speck cases I had on previous laptops and I was only using the bottom of those as well.
So my personal belief is everyone should have a sleeve, it can be cheap investment. Hard Covers take away the simplicity but bottom protection is nice. Skins are just Artistic expression.