Brilliant answers. Sort of how I expected but love seeing peoples personal choices.
I'd looked at this as if that bike I had to occasionally commute on or hit the road, which is fine as I've seen guys round here off to work on their full sussers, proper things too not BSO's!
I chose the Day One as It's dead light and can do everything but moves so easily and is springy and playful, but it could easily be my Cotic Bfe, though if I only had one I'd size up and lighten it up and get a Soul or maybe a 29er/650b
It is right, if you had one bike, maybe due to space or money or rules, it totally would be 'Go go Gadget' Bike, loads of swap out bits. One frame, 2 sets of wheels, 2 sets of forks, other bits n pieces. That works great. To totally blow it you could actually have a spare frame
but that ruins the exercise slightly, and is how we got in trouble in the first place!