Hay, Umbran, ya I have heard that too (from you and others).
I have had four cats in the past 18 years- one died from stomach cancer, and the other from a heart defect. The other two like to go out in the morning, and once or twice during the day.
As far as enriching the interior environment, I would like to hear more, as I would like them to make the choice to stay in more, but I want them to have an understanding of the world outside, to not wonder their whole lives- "what's over that fence?" Instead they know there is a big dog on the other side that doesn't like them.
I am very much about freedom, choice, thoughts, and actions (so long as it harms no one else), so keeping them locked inside bothers me.
Our cats have the choice to go out, sometimes they do and sometimes they do not, and some times I chose for them (one was sick last night so he stayed in today).