All mine had dummies once bf was well established, it saved me bf constantly just for comfort.
I agree with those who say, dummies for older toddlers are fine for naps and comfort when upset but not to have them stuck in their mouths all day. I used to have to hide my kids dummies and keep them out of reach, because if they saw them they would have shoved them back in, but I would say it was very noticeable that they didn't talk as much with them in which I why I wouldn't let them have them all the time.
I teach pre-school classes, and one 3 year old little boy has never uttered a word in my class because he always has a dummy in his mouth. Obviously he can talk and will learn to talk properly, but it may well be delayed compared with peers, which at school age (only 4, remember) can then have a knock on effect on phonics and reading.
They also don't look very lovely I agree, but that would be a secondary reason for not having them in all day not a primary one.
I know a lady who has a 3 year old who still bf, I don't judge her for that, each to their own, if the child still wants it that's fine. However, she is basically used as a dummy, every time the child is remotely upset or bored she demands boob so the lady is constantly in and our of her bra (eg about 6 times during a 40 minute period I was with them). While a nice thing to do at bedtime or similar, being a human dummy jsut strikes me as annoying.