I also think that pg/vg should be determined by vendor for best experience.
AiV does that, they are selling their juices mixed to what they are sure is the best ratio. And you can request another pg/vg ratio, you'll get warned by them:
But you can do that.
It's like restaurant to me, I'm fairly sure that chef knows how much and which ingredients to use in that dish for best flavor.
But the vendors don't know what your vaping with. Some delivery systems have a hard time with a dominant VG profile. Some people are allergic to PG.
There are so many personal preferences that using that method could very well drive a lot of business away.
Wlad @ Ahlusion's actually encourages you to email them and say what setup you are using so they can give suggestions and personalize the liquid to not only your tastes but equipment for best performance.