I don't base my purchase 100% on a paintkit, but if there is one, and it is good, then it will influence heavily.
I almost don't fly the sim at all anymore, most of my fun comes from painting. But, I don't have a lot of time for hobbies, so if a good paintkit already comes with a package, I can spend more time just painting straightaway instead of making paintkits.
Of course, if a product looks great, and it's a plane I like, then I will buy it, even if there's no PK. But, as a painter, a good PK is a big boost for me, and makes it more likely that I will buy a product.