Usefulness of the ISO dial?
I'm the big fan of the ISO dial. It's very useful when shooting in manual mode especially at night when you want a precise exposure but the camera is not smart enough to choose the correct ISO. That's when the ISO dial becomes really useful because you just have to go through the different ISO settings to get to the one you want.
With no ISO dial, you might be able to set it to a function button, but it's never going to be as convenient as an external dial.
Another advantage of the external dial is you get to see the settings immediately without looking at any screen.
The Fujifilm X-T1 came with a lock on the ISO dial. There are those who complain about it. Well, there's a lock on the shutter button too and have you been using it often enough that the lock becomes a nuisance? Are you going to change ISO that often until that lock becomes a nuisance? Depends on your shooting style of course.