Just so you know, they will all crack over time, and sooner than you'd want them to. In my many years of MacBook ownership, I've tried so many brands over the years, and hard shell plastics/polycarbonates just start cracking, snapping, and chipping, and it looks ugly after a while. I baby my MacBooks, and they still start to crack within a month or two.
These $30-50 cases by Incase, Speck, Moshi don't fare much better than many $10 cases from companies like iBenzer, Mosiso, or Fintie on Amazon and eBay. They're just branded and sold more commercially, but have barely any more rigidity, durability, or made with slightly better materials or quality assurance.
Why companies don't use soft or flexible TPU material like they've been able to do with iPad cases is beyond me.
The only one that I've been monitoring over the past year that seems to hold better than any of these is from a company called UAG. It's polycarbonate with rubber molds around the vulnerable edges and sides. Granted, it makes the MacBook a lot thicker, but reviews are coming back very positive in terms of durability (no cracking or chipping) while providing actual military-grade drop protection.
They're not cheap, mind you - averaging $70 on Amazon and eBay, but if you don't want to spend $10-50 per year in replacement cases, this may be a better investment.
Do a search for "uag macbook 12" on either site to see what they look like and cost.
Update: if the UAG is too thick for you, another brand I found called STM makes a hybrid slim case called Dux with rubberized TPU protection. Might hold up better than those other cases without it being too bulky like UAG's case. https://www.ebay.com/itm/172125086803
Keep in mind the bottom of the MB is where the CPU passively cools through a graphite sheet. Adding an additional layer of plastic only insulates. I think snap on cases are a horrible idea in this case.
1. Apple purposely designs its case for cooling (as previous poster mentioned).
2. Why cover a really wonderful look.
3. The design is for slimness so why make it thicker and bulkier.
4. To each his/her own.