I wondered what that contraption was! We use ours for carrying stuff around our house and holding boots, etc. during the warm season (all 4 months of it, HA!), so ours never get heavy enough for a cart.
I could see how it would be useful, though, if you were using the buckets for their intended purpose.
I've got a manure cart for that, the muck buckets are a pretty inefficient way to move poo, but they didn't ask me! - I use them for water buckets!