Oh, the TRACES! Yes, that's a definite no-no. The original (ADS, not show) reason this is a bad thing is that it negates the effect of the single tree. Instead of allowing the harness to move with the horse so he can pull and bend properly, it causes one of two effects: either the breastcollar slides around on the horse causing chafing and discomfort, or the horse learns to lock up their shoulder and short-stride in the front end because there is no freedom for them to move their front legs out of the way of the hind during proper engagement. Bad thing, very bad. Like most traditional problems it doesn't show up as much on a mini (especially with a light show cart) as it would on a big horse with a heavy wooden cart, but it's still true.
Most of the mini show harnesses are very light (so wouldn't chafe much) and the traces are soft enough they just drape down if they aren't tense. But synthetic harnesses or thicker traces on a carriage harness are stiff and they do push the breastcollar forward (meaning the harness stands still while the horse moves- causing chafing) if the ends of the traces are attached to something stationary instead of a moving swingle tree. Bit more of a problem there.
Leia