normally You will take it off. When You fly in Sport-mode/snowy/rainy conditions the dome will enhance aerodynamics and prevent insects/snow/raindrops from blurring the camera. Disadvantage: once the dome is scratched footage might be blurry and when flying in bright sunlight the dome might cause unwanted reflections.
For my part I made this mod for the gimbal cover : Gimbal cover mod
No more lens flare/glare and noticed no reduced speed with or without my modified cover.
Not confident yet to fly it with snow/rain
Made my first flight yesterday with the mavic. Went really well and no issues had. I did leave the gimbal dome on... and yes I could tell it was on there from the footage (probably didn't help that I realised afterwards that I had fingerprints all over it). It wasn't even sunny and I could see some lens flare/glare
The footage was still pretty damn good though i have to say. if you want to make a flawless film, take the dome off id say.
It shouldn't over heat when left on , but you will almost certainly have reflective issues, it's good to keep on for beginners or those testing there Mavics, but for shooting photography leave it off
Leaving the cover on shouldn't make it overheat. There are openings in the dome to allow it to still take in air. DJI staff on other posts have endorsed that the dome can be left on because of these openings. Your certainly not going to get the same quality of footage with it on, but it gives that extra level of protection to the extremely delicate gimbal and camera.
In the height of summer on a very hot day.... maybe id have second thoughts about leaving the dome on due to air issues (but thats just my personal opinion)