I've found that my laptop runs a bit cooler not running clamshell mode. Quite a bit of heat is radiated through the keyboard - coconutBattery and iStatMenus seem to indicate this to me. (2017 MBP 13').
I use my MBP with a 32' 4k monitor at my house and when I used to use it for work, 2 27' monitors at work. I don't understand the guilty feeling. Having screen space to use with your device is an amazing thing. I love having 1 computer that has everything, and I can use it anywhere.