@rvaughnmd You can't control for the wind - no posture improvement. You can't improve your handling, either. Indoors will not prepare you for 100% outdoors riding.
And outside the few people of huge discipline who only need the training, you have the people who need extra motivation. Some psychological aspects can't be replicated. The smells, the sights, the variety that comes with unpredictability, the taste of fruit on the tree when you decide to pause.
And have you seen the snow today? Woo!