I love the simplicity of Rbenv. I also love the "it's never good enough" mentality. I have not had any issues with RVM, but that doesn't mean people like Sam shouldn't try to build something better.
I think this quite reflects the whole evolution theory - something quite good is replaced by something a little bit better, which is again replaced by something a bit better, and so on, and so on, and so on.
It more so to me reflects the knee-jerk reaction of many in the Ruby community to jump to newer projects simply because they are newer. I like rvm a lot and use it all the time. I'm sure rbenv is a good solution as well. I think it's a bit premature to say it's an overall better solution.