Why use embrocation versus just oil?
Interesting question! Embrocation sounds very gimmicky to me but I don't race. I wonder if the effect is like drinking hard liquour when you're cold, which a lot of ppl think warms you up. In reality alcohol is also a vasodilator, pushing blood to the surface of your skin and making you feel warmer. This is contrary to what your body is trying to do in cold weather, which is maintain core temperature. 

So arguably, vasodilation could be a good thing in a controlled and not-too-cold scenario, e.g. cycling in cold weather, where you want your muscles and skin warm, and are not concerned about hypothermia. But it's probably to be avoided in seriously cold scenarios, although at such times you're probably no longer relying on embrocation for warmth but instead full-on thermal leggings or snow pants.