I use a santoku for almost all of my cooking.
The reason is that it has a high wall good curvature from the tip to the mid and a long flat. This complements my cooking technique quite well. I am able to move quickly and efficiently. I find a Gyuto of the same size useful but the long sweet and short flat does not work as well for me.
I like using nakiri but for different tasks like match book fry cuts for example, that being said I am not as quick using my nakiri over my santoku.
Even when I was doing work in professional kitchens I found my self grabbing for my Santoku over any other knife.
But that is what is beautiful about knives, they are an item that is personal and people find preferences in some designs over others.