It's not that I don't grasp the concept. I just feel like it's adding too many extra steps to what's as simple as math? ... probably the same reason why I (and DS5) have serious issues with Kindergarten homework. It's too much hands on and not much math that would move us forward.
This was the exact problem that DD had with math during preK through about 5th grade. She has (maybe always??) thought abstractly/symbolically without needing manipulatives, so to her, the entire concept of a physical, three-dimensional representation felt weird and unnecessary-- and it freaked her out and made her feel like "oh, well, what the heck is THIS for??"
I still remember how fascinating it was to my 13-yo self to learn binary and base-8 math during my first forays into computer programming. It was AWESOME.
My DD thought I was hilarious when I sang Tom Lehrer's
New Math...
(He discusses regrouping in another 'base' system towards the end of the song. It's very charming/funny.)