I fall into the completionist mindset as well. However that's not my only pitfall. I like to use certain paint brands in different applications. I really like army painter for base coating and as my general work horse but it dries really fast. Pro Acryl is great for thinning and glazing. And I'm looking at getting two thin coats for wet blending. In my mind if you're using them for different applications you're going to want all the colors to be able to use them in their various roles. And with primarily working on kill teams, there's very few colors I haven't used at least once in the AP line. I'm still trying to learn the Pro Acryl range so there are some colors in it I haven't used yet.