In fact, it's well-documented IN THE CREDITS at the end of the show. The assistants are LISTED BY NAME. Trading Spaces never claimed that ONLY the couples did all the work, and from the scope of some of the work required on some shows (huge areas painted with multiple coats) it's more than obvious that we don't just have a guy and a gal and a couple of paint brushes.
The editors only show the couples and the nominal participants (Paige, the designers, and whomever the carpenters des jours happen to be) because those people make interesting TV. The off-screen helpers (who mostly assist with "homework") are there to make the show look good.
"While You Were Out" could USE some more assistants, because a few times they've come up short on man-hours and some projects have gone unfinished. Both shows are cool entertainment, though, and you can easily pick up some great deco tips.
Michael