I've been thinking about this since you posted it, Angie. I don't mind wearing gear as casual wear at home or out for everyday errands, and I think it comes down to a few factors:
1. I read a study that said that people who wore more athletic clothing during the day were more likely to squeeze in unstructured exercise. Because I prefer that to fitness classes or regular morning runs (ugh), it's important to me to feel comfortable for a mid-day walk to the post office, an after-dinner game of Frisbee, or a quick hike through the trails at the sculpture park next to the mall.
2. I live in a small town, surrounded by nature. If I lived in a city center, I suspect I'd be less likely to feel comfortable in gear. Here, it suits the environment and the environmental norm. People are used to stopping off for ice cream after a hike or riding bikes to the cafe for lunch, so outdoorsy activity is built into our lives.
3. My gear silhouettes aren't significantly different from other silhouettes I wear regularly.
4. I work from home, so my days are varied and unstructured. Changing clothes per activity seems less practical when the activities are jumbled together willy-nilly.
(ETA: I missed Una's response earlier, but I agree with this 100%:
"It's part of my lifestyle to be prepared for any opportunity to be outside. Plus I am not a disciplined gym class person at all, so I don't have defined moments of "yoga time vs. rest of life". But I may hike in the mountains, then go to the Rack or dinner or hang with a friend on the deck, all in the same outfit."
Yes!)