This question always seems to come up, and there have been some really interesting arguments on the CPUvCPU side in the past on these boards. I can tell you that I love CPUvCPU for dynasties for two main reasons:
1. I enjoy sports games for the very reasons I enjoy sporting events. The outcome is usually in doubt, and the focus of the game is the strategy and performance. Many people enjoy taking control of every player on the field, and for me, that detracts from important ratings like Awareness, etc. This also leads to an abuse of the AI and gameplay limitations that will always exist. Rather than "cheat" the CPU, I like to let the "players" play out the strategy I construct.
2. I also don't have the time to commit to sitting in front of the TV wholly encumbered by a game for an hour or more, but I still enjoy the fun of managing a team through a dynasty. I can still recruit, set depth charts, and generally set playcalling strategy. Of course, I am a former thumb jockey (and guilty of cheating the CPU on many ocasions) and I can honestly say it is a lot more rewarding to win a title in Coach Mode or AD Mode, where I can't exploit defeciencies in the programming. Some guys win 8 NCs with some obscure school and call that fun. I rarely win a NC with a power school, and I can't wait to try again next year. I call that fun.