The only reason would be the usual, why not? i switch it up every few years. I finally settled on 9mm and 45 caliber, after almost 4.5 decades of carrying. So perhaps the XDS could take the place of my 26 or M&P and keep the 2 calibers I use in the same model gun instead of Glock, Smith, and Springfield. It would be very efficiant, but I really like glock 26's.
I have to shoot one and see if I can simpify life even further. I recentlly got a bug for a revolver, after finally getting rid of my last one a couple years ago. That's what make it fun, if we kept the same thing forever and never switched it would get tired. I just like to find the newest best gun that comes out for my own gratification, and the most effeciant means of protecting myself and others.
I started with a Dective Special in 1972.
I was one of those people who had a 26,27 23, and 19. Then slowlly got others that evolved into the ones I have now, it's a constant evolution depending on what comes out and where your head is at when it happens. I never thought back in the 70's that I would own guns that held 18 rounds either. The single stack Walther and Colt were "it" at the time. But things continue to change in this sport/lifestyle.