Yes I agree, air will rise to the highest point it can. But in my opinion having the air-trap at the highest point is MUCH harder!
In my "low-rider" cube case it's very hard to get the kind of layout that you have ECUPirate. As you mentioned, good water-flow will flush the bubbles out of most systems, and that works great for me! I previously had a problem as I had no air-trap and relied on the bubbles to rise into a perpendicular fitting of a "T", filler/airbleeder setup. It simply was not effective enough, the flow in the line was so strong that it swept the bubbles past the "T"-fitting. With the air-trap in place, I fill the system in just a few minutes and the bubbles are all gone within 15-20 minutes.
I'm just waiting for my heater core now
__________________
Pump:Eheim 1048,
Radiator:LPD radiator 100x150x20mm,
CPU block:DD Maze-2,
GPU block: DD GPU block,
Chipset block:Zalman NB32
AMD XP 1500+, EpoX 8K7A, PoV GF3Ti200
Pic of my YY-cube