This could end up being a great move. It's a friendly deal, adds a veteran with championship experience and a guy with whom Cassell has a history. He's not the defender that Ariza is but he still has the range to preserve out spacing and hit those 3's on kickouts from Wall.
I think the most important thing is that he can create his own shot in half court settings. Only Wall (driving to the hoop), Nene, or Beal could really generate their own offense reliably. When things slowed down late in games in the Pacers series the offense totally stalled. Having a guy like Pierce will pay huge dividends in those situations. Even if he only averages 13 ppg, he's still a SHOTMAKER.
Not to mention, IF Otto can continue to progress, he'll be able to transition into the starting role as Pierce declines.