Do cases hold heat inside a macbook?

The entire aluminum lower case is a component of the machine's cooling system, absorbing heat from the very hot internal components and releasing it by convection to the air outside and by radiation to other things outside. A plastic case or skin of the sort you're considering impairs radiational cooling and prevents convective cooling from the underside of the machine, forcing the single internal fan to work harder (i.e., spin faster and more noisily) to keep the machine at a safe temperature, and also causing the keyboard and wrist rest surfaces to be hotter than they otherwise would. This will be OK if you aren't pushing the machine hard, but if you are encoding video, playing demanding games, watching a lot of Flash video on web sites, or doing anything else that loads the CPU heavily, the computer may get uncomfortably hot, and may even get hot enough to shut down for its own protection.

In short, I don't recommend using such a case.

Yes!

I just found this out last night. I bought a new 2017 MacBook Pro with touch bar and was using a plastic hard shell case for the first two days. My laptop got hot just doing basic things like typing and browsing the internet. I took the shell off and computer was cool the rest of the night while I watched a movie on Netflix. It is definitely the case. I bought it to protect my laptop from scratches and dents, but if you think about it an 8 dollar plastic case is not going to protect your MacBook from a fall off the table or a drop on the hard ground. Just be careful and by a protective sleeve for transportation you should be fine. Let me tell you all -it was a huge relief to find out that the heat was being caused by the plastic case and not the laptop or software update.

if your worried about scratching or damaging the aluminum of the MBP i'd just invest in a good carrying case and use that when you move it around and leave it au natural when you are using the laptop

The shell definitely affects the cooling. I've experimented. My mac heat up and the fan was blazing like crazy. I removed the shell and it stopped immediately.