I think it's absolutely ridiculous. It's probably more for the mother's benefit than to appease any desire on the kids part. I don't know any toddler who wants to do something a "baby" does. They are generally adament about being a "big kid". As a toddler the kid is old enough to remember suckling their mother's breast....that's got to screw with them as an adult?
I had a friend who breastfed her children until they decided to give it up. And I distinctly remember being at her house when her 3 or 4 year old came up to her and started tugging on her shirt because he wanted some. I don't necessarily think that they understand this is usually only what a baby does because it is what they have known all their life. I think it gets harder for them to give it up at that age.