On topic, I too agree with the folks for whom solo for river crossings are a rational fear. I had a friend die under analogous circumstances, and on a trip I backed out of at the last moment. So solo, slips on bank, hits head on rock and knocked out, ended up face down in shallow water, drowns. With a partner maybe a good chance to avoid the last part by getting pulled out/flipped over. I agree it sounds like a bit of a stretch, but apparently this scenario happens all the time according to the ranger I dealt with when my friend went missing. The hypothermia is an another, since if you reach the point where you loose good control of your limbs, and/or your thinking, having a someone else there to be you surrogate brain and hands for a time can save you life.
On the other hand it doesn't stop me, and like others have said, I enjoy the added level of attention I pay to things when solo. Like a lot of things, the level of mental pressure is if not pleasurable, then fulfilling, but as it is ramped up there is a point where it become too much. For example, at least at my level of skill and lack of grace at times, if I tired to do parts of the High Sierra Route solo it would simply become mentally debilitating, and I think for me that also would be a useful and wise fear. Not saying it wouldn't be very useful exercise in self knowledge, just probably too much for me mentally to consciously seek it out. I'm no Aron Ralston.
I realize a lot of high level climbing, for example, is about rationally approaching this edge and pushing a bit past each time, and that some people feel this process is the most meaningful thing they have ever experienced in their life. I can believe it. I just feel like playing with that edge is not for me. I'm happy to look over the rational fear cliff from a safe distance rather than to intentionally dangle my legs over the edge.