If she doesn't dump her boyfriend for you, then she was worth it but is unreachable.
If she dumps her boyfriend for you, then she'll dump you for the next guy too.
Lose/lose situation. Not saying anything means you avoid that lose/lose situation and aren't a thieving weasel: win/win situation.
That's what I was trying to say before I gave up and went to bed

.
The only way you can get a 'win' out of it is if you're only after a one night stand, and even then it's still very bad manners (and could cause long term damage to both the woman and the other man).
Yup. Any girl who leaves a guy for you is just as likely to leave you for anyone else.
Unless the bloke she's leaving is outright abusive (and you aren't). But if that's the case, she probably shouldn't be jumping straight into another relationship.
(it's fair game as long as they are not getting married or are married)

Given the ever increasing number of people who have relationships and no intention of ever exchanging rings, who are more committed than most marriages, I don't think that holds true at all these days.
Also, one element that people seem to be ignoring is that the women involved may find it annoying to be hit on by men who know they're taken. Flattering as it may be to feel desirable, having someone pursue you despite being aware of your relationship is insulting; it implies that you would betray your partner.
EDIT: We should all bear in mind that we don't all move in the same circles; girls/boys who go out dancing and drinking every night are unlikely to react in the same way as a retiring P&PRPG player (and even less extreme examples can make big differences).