I think consideration for others is tricky now a days.
Today, you see people constantly talking on their cell phones, even when they are spending time with another person.
How many times have you gone to a cafe or even a restaurant, you see two people at a table. One person fidgets, looks around, bored.... because the other person is busy talking on his cell phone.
I rarely ever hear, hi this is (name) speaking. I'm sorry I'm with someone right now can I call you back. No its gab gab gab, so the other person sits for 5-10 minutes feeling completely ignored.
I have noticed if I am with a girl, she will tend to either shut off cell phone or say not now. Of course the girls I choose to hang with aren't the type who think cell phones are their best friend. (Ex. See phone as a tool, not as the latest fashion statement)
I have noticed with guy friends, they tend to ignore you and talk on the cell phone.
I guess if socially we are moving towards where consideration is not a factor, then why not. Use your open air headphones in public.
For me, if its an active area. Meaning there is a lot of conversation around, like on school campus at local student hang outs. I see why not, its not like I can't hear 50 different conversations going on.
Although if I am in a quiet situation, at a library, I'll take off the open air and listen to it off my head and adjust it until its a level that others around won't be disturbed. Plus while I'm studying, I prefer my music low. I want music, but I just want it background. I'm sure you know what I mean. But I'd prefer to have closed headphones in this situation though.