I wear flip flops in the summer (and am unapologetic about it). They aren't the flat $2 rubber kind, but are nicer Reefs with a platform (so my jeans don't drag on the ground, since the only other shoes I wear my jeans with are always heels). I wear them when I am dressed casually and doing casual things. I will also sometimes wear them when walking to and from work if I have stilettos on that day, because I don't want the heels of my shoes to get caught in the grates as I am walking (which completely ruins them).
I also go to the drugstore in my gym clothes. My gym is two blocks away and the drugstore is on the way. I'm with Maya and think it would be ridiculous to avoid going to the drugstore simply because I am in gym clothes. And considering the fact that when I go to the drugstore in gym clothes I am sweaty, I think it's pretty clear I have come from the gym.
Angie, I know you say that you can't condone gear to do errands because it's a slippery slope, but IS it really a slippery slope? It seems like everyone here on YLF who wears gear to do an errand has specific instances in which they do so, and has strict limits on it. In that case, it seems that everyone is able to limit their gear-wearing, and there is no danger of sliding down that slope. I think that should make you feel better (and in my opinion, makes these isolated instances of gear-wearing okay)!