On easy runs it doesn't matter being up high on the back of the boat; it is a nice view. When you get into good class IV and V, it is very much an issue. You have to be in that seat, where you can easily get launched, to have any control of the raft. With a paddle in my hand, I can be farther forward, or down low and still have all the control I need. I can jump over to the left with my paddle and highside with my paddle in the water. After getting down, I don't have to climb up to that vulnerable position and get both oars in hand to have control. If you run a slantboard, you can't really get down at all.
If you know exactly what the hits are going to be, sure you can ship one oar, ditch the other, and then hold on with the free hand. Or if you have huge mitts, hold both handles with one hand--that's what I do if the channel is wide enough. But that also means you know the run really well and might not be getting on enough new rivers. Also, if you have a full raft, letting go of those oars is the not best thing for your passengers' heads. Personally, I prefer a strong paddle crew, and a paddle in my hand if I'm dropping difficult, new whitewater. Or give me a real frame and oars--I could still put paddlers up front, or even in the back.
To really answer your question, you need to determine your POU--Philosophy of Use. If the POU is day long float and bloat, definitely buy the stern frame--your "paddlers" will love you for it. If you think the stern rig is going to make you more comfortable running harder whitewater, I think you'd be better off finding a solid crew.