Stern frames were developed by commercial raft companies as a hybrid oar/paddle rig, especially for high water use. I believe stern rigs were required as part of the agreement when the Shoshone run was being developed for commercial trips in the 80's, particularly for the higher flows. Govt types believed passenger safety was enhanced. Guides had a little different gear ( no self bailers among other things) and less experience then compared to what is out there now, and I don't think they are required any longer.
They are somewhat clunky to drive, as the oars are off the stern far from the center of the boat, which causes a lag in response time for lining up the raft, which also leads to more corrections (read: more work!). They are also not very versatile, although they can be rigged as a center mount. But the geometry of the rear end is different than in the middle of the boat, so this is less than desirable as well.
An experienced paddle guide will be able to get a crew down class II with little effort, and the well-coached paddle crew really only needs to work in rapids and occasionally to keep a line in the main current. A boat driven by a paddle behaves different than a stern rig, the trade off is lesser performance for higher confidence.
If one is concerned about their paddle-guiding ability, or if beer is a priority for guide and/or passengers, just take the center frame and give them a couple paddles to play with.