Because the air pockets get trapped in one place and cause hot spots, if that air pocket is at the t-stat it can stop coolant from reaching the t-stat and if that happens the t-stat will not get hot and open to let coolant through and the engine will overheat.

Take a brand new shampoo or soap bottle, one you can see though, tip it upside down and watch the air bubble, it moves VERY slowly to the other end of the bottle.
Now take and empty half of that bottle and try again, it all flows very quickly from end to end, the coolant will do the same thing.
Make sense?

 

Is the normal trap area the heater core? Does the trapped bubble move if RPM is higher (more pump circulation)? Doesn't the "jigjle" gizmo on the stat allow gas to pass by and prevent the "steam bath" scenario where the stat is not being heated?

 

Air can get trapped anywhere.
Yes higher rpm can move the air.
I have no idea about the t-stat gizmo thingy you speak of.

 

He was running pure Tennesee Sour Mash, it only needed a little bit.

 

Some came with a little hole drilled in them, now it seem to be popular to a have a jiggly wire in that hole, it is normally installed with this device at the top of the stat (if in a D1 for example). The idea is that it lets gas bubble pass by the stat, so body of stat can stay in water and work at correct temp. Pix shows one at about 11:00.

 

But how many people know that hole is supposed to be at 12o'clock when installed?

 

Put that in a sticky about the thermostat. I have seen it on here before in some thread.