Those are good points. I don’t have any real world experience to back, just what I’ve read. This guy did it with a Mostly Printed CNC machine (which I would imagine is a little looser on the tolerances than the Shapeoko) and off-the-shelf GT2 belt:
https://www.v1engineering.com/forum/topic/mpcnc-with-double-belt-setup/
I think the advantage is that you could increase your feedrate (say, in a production type situation) without worrying about belt stretch or losing as much accuracy in the stock configuration. It also might lead to increased repeatability. Also, I think it would help if you were considering extending your rails. For example, if you had an 8 foot bed, belt systems have a clear disadvantage compared to lead screws or rack-and-pinion systems. But the cost goes up dramatically. What if you could get comparable performance to a rack-and-pinion system, but with the low cost of a belt system.
If I ever get around to doing this mod, I’ll let you know. 