I've bought an X-T1 + a 23mm as a first unique lens. I'm still waiting my parcel so my testimony is worth nothing but I'm quite sure that it'll be more versatile than one single 35mm. I have an EF 50mm 1.4 on my FF Canon 6D and I mainly use it for portraiture, or night photography. Most of the time, I feel it to be too much narrow and not confortable specifically for indoor shootings.
As I was looking for a light kit and a 2nd body, my objective is to have only 2 primes lens on the X-System : 23mm 1.4R and 56mm 1.2R.
Of course, I still need to make savings for the 56 1.2, so I'll do my first steps only with the 23mm and it could be a good thing. Indeed, I'll be focused on framing and not switching from a lens to another as I tend to do with my 6D and the 17-40 f4, 24-105 f4, 50 f1.4, 135 f2 in the bag.