Here in Thailand we have an abundance of amazingly beautiful girls that love to pose. Needless to say I shoot at least 1 weekend a month, maybe 4-5 models each time. I always use a light modifier. Typically I have a yn-468 on a light stand with an umbrella as it's light and portable (a huge issue when the average ambient temp is 36c and I have to trek to the other side of town via public transportation). I do use ambient lighting in some shots, but I always carry and use the flash/umbrella.
So to answer your question:
Yn-468 with umbrella - 80-90% of all outdoor portraits (but with me for portraits 100% of the time)
10-20% natural lighting, with that highly leaning towards 10%.
I like to use an umbrella as just a flash on a stand is a bit harsh. Umbrellas are cheap, light and portable.
I really can't see shooting outdoor portraits without at least a flash. When I first started shooting I tried just fast glass, but I was disappointed with shadows and lack of pop.