My own opinion is that cooling holes mostly just let a motor cool faster once you have stopped. This can save your halls. At that point the pattern does not matter so much, once stopped the low holes suck air, and the high ones vent it out. I do think you should make your holes so you can see windings through them. That's the hot part after all. And you can monitor their color over time.
Pushing the motor that much beyond its rated wattage, gets you into more trouble than its worth, but cooling holes might have specialty utility, such as racing situations. When I pushed a motor too hard, cooling holes made a nice display, as the flames shot out the holes.
Yes, they let in everything, dirt, water, whatever is out there. The one good thing I noticed, was that while briefly paused, I could tell the temp of the motor by smell.
But the bottom line, don't push any motor too far. trying to run 4000w through a 500w motor was too much. Those same cheap dd motors behave well when given 2000w though. At that point, cooling holes are not needed provided the motor is not seriously overloaded.
