If you put the Denver stuff and even the Spencer injury aside... Ron Dayne is simply not a solid, dependable, NFL back. Look at his stint in Denver. He was a hero last year against Dallas, and was a viable option through the rest of last season. He came into camp with the Broncos with a shot at the starting job. Early on he was actually favored to start over Mike & Taco. Then somehow he fell behind those 2 and Cobb and eventually was cut.
That tells me that he was either too fat, too slow, too lazy, too injurious to teammates (sorry, had to).... or just plain not good enough to cut it.
I think Kubiak brought Dayne in because he was familiar with him. The sooner he steps back and looks at Dayne for what he is, the better for the Texans.