The only reason I would approve this deal would be if it was the stepping stone for subsequent moves this summer (or even tonight). Adding Mo gives you the flexibility to move Harris - which would both lower team salary and not force Utah to take on any extra contract years - a huge plus in my book.
On the court - it would also give Utah the flexibility of playing 2 PG's in more effective stretches to match up with an opponents speed, and despite Harris' a career year shooting from behind the arc, Mo is definitely a better 3pt-shooter than Harris in halfcourt situations. I don't see any upgrade in terms of distributing the ball, and durability-wise neither Harris nor Mo is particularly an "iron man."
Simply adding Mo along with Harris to tagteam the PG position for 2012-13 gives Ty another toy to play with and more options in the backcourt, but overall makes little sense it in the grand scheme of things, and I would be surprised if Mo agrees with it seeing as how he could easily be a backup PG/SG for a more successful team in a bigger market. He'll be 30 at the start of the season and knows this is his last chance to secure a multi-year moderately handsome contract so I expect he'd prefer staying in LA over being PG 1B in Utah.