if you look at a standard pole piece, the coil rides along the outside diameter of it and the pole vent blows through the inside. to me thats not a great way to cool the coil.
backplate gap cooling imo is a better method vs a standard pole vent
some manufactures like dd even have holes in the former to bypass airflow through it and the top plate channels. in their application the pole vent actually aids in real cooling abilities. this is a prime example of pole venting aided by other methods together can be beneficial for cooling
obviously all manufactures approach it differently, but a standard old style pole vented motor from a classic standpoint is not efficient by itself for cooling.
on the other hand as said already, pole venting is less motor material, thus a lower bl and slightly weaker motor