MY answer is the it is okay. I know there are others who would disagree and you are welcome to chime in your opinions as well
But for us, we do and I don't have any regrets doing so.
There are times - especially when he falls or gets hurt or he gets in trouble and disciplined and then wants some hugs that he also wants a little breastmilk. For him, it is totally the comfort factor and not the actually milk at all. I think this is part of the design of breast feeding and I personally have no problems doing it.
That being said, there are times as he is getting older and he is just bored and wants to nurse that I do say no and provide something else for him to do or his sippy cup. You know your child and you know what is best for both of you!