I have never seen such a thing in my life. The only place I have ever even heard people talk about a woman letting her breasts hang out while nursing is on the disboards.
I have known many people who are extreme proponents of breastfeeding and was even a member of a holisitc moms group and have never seen someone breastfeed "indiscretely"
I will preface my remarks by saying that I am a nurse so there is not much about the human body that shocks me in terms of seeing body parts. I just generally see them in the privacy of someone's hospital room and not in the checkout line at the mall.
I will introduce you to my crazy SIL. She breastfed her children by removing her breast entirely from it's nursing bra cup, holding it in her hand with the entire breast completely exposed to "adjust" it (I never understood what there was to "adjust"...it only hangs one way. It's not like it has a twist off cap or something!) and then would make a big production out of saying "Oh, Baby is hungry today, he's really latching on, what a good suckler Baby is" (OK, thanks for the play-by-play Howard Cosell!

). She would switch sides by leaving the 1st breast hanging out of her shirt and doing the same "maneuvers" with the second breast...out completely, "adjust", play-by-play, with the first breast still hanging out completely. She could clear a room faster than lightning...the moment she started to unbutton her shirt....VROOM! everyone was gone.
The other indiscreet BF'er I saw was the lady whose
5 year old came up to her at a New Year's Eve party, unbuttoned her blouse and started to BF. Now mind you, all the men in the room went RUNNING to the bar (which was, thankfully, in a different room). She was another one with the breast hanging out thing. I don't think there's anything worse than seeing a breast entirely hanging out with a
5 year old attached to it.

I am also not sure of the wisdom of having a much older child (like a 4 year old or older) BF....I think there could be some developmentally "confusing" issues there. It's been a long time since I took child development though.
All that being said, the majority of people I know BF just fine and perfectly normally and discreetly. But it's always the odd ones that you remember.