Think of it this way: The 49ers aren't going to trade a starting-caliber quarterback to a division rival, but if a division rival shows interest in Smith, that could help San Francisco leverage better trade terms from another team.
The 49ers are seeking to trade Smith to avoid guaranteeing $8.5 million to him in 2013. Trading Smith would beat releasing him for reasons beyond trade compensation. A trade would also give the 49ers some control over where Smith played next season.
While the 49ers wouldn't necessarily fear facing Smith in the future, NFL teams aren't in the business of helping their division rivals. Trading Smith to the Cardinals would upgrade Arizona's quarterback situation. It would help the Cardinals even if Smith played at only a mediocre level. That's not very appealing from a 49ers perspective.