I think it depends on the couple, particularly the person being proposed to. If the couple has previously discussed marriage (i.e., the proposal isn't just coming as a total surprise to the party being proposed to), and the proposer knows that the proposee is a) certain or very nearly certain to say yes and b) that the proposee enjoys being the center of attention and would not be embarrassed (or would be pleasantly so), then I think it's fine. Of course, the final caveat should be that the proposal should not distract from anything else major going on. A big public proposal should not happen during, say, a theatrical performance or a concert, and it really, really should not happen during something like a wedding reception, baby shower, or bar/bat mitzvah party. Don't take away the spotlight from the person or people being celebrated on that day.
Personally, I would hate a public proposal. I hate drawing attention to myself in public, and would be unable to enjoy the moment for the embarrassment. If BF tried something like that, he would be in very major trouble. Fortunately, that is not either of our style.