You can't look at a trade for AR including Bonner as a one-for-one. As in, you lose one and the other's supposed to step into and fill his role.
What you're doing is acquiring a player that may or may not do much if anything this year, but just may turn into a real star down the road -- given the quality of the franchise he'd be coming to. Getting rid of Bonner would push Splitter up the depth chart, and that's why I'd do it in a heartbeat.
What Bonner does, as fruitful as it may seem during the regular season and in a +/-, is not something the Spurs have relied upon in the past to win championships, nor should they rely upon it now to win championships. They don't need a 3-point-shooting, one-trick-pony 4 -- they don't have the rebounding or shotblocking at the wing to compensate for it.
I've said it before and I'll say it again: a championship-caliber team needs specialists, so long as what they specialize in is what the the team lacks. Bonner is their only 3-point shooting 4, true. But losing him won't lead to this team lacking when it comes to championship aspiration.