there is no reason to-- it doesn't help your bond and it is full of extreme risk for the baby. Tiny mistakes cab be major, including being a few degrees off with their food, not properly sterilizing surfaces/syringes due to cross-contamination, aspiration etc etc, Sour crop , bacterial infections etc etc. If one has apprenticed and shadowed and done this under supervision with an experienced hand-feeder and if one has a brooder etc, the it can be done, but there is just no point unless you yourself have extensive hand-raising experience...Watching the breeder hand feed a few times doesn't count, btw (breeders sometimes push this because it makes their lives easier).
http://www.parrotforums.com/breeding-raising-parrots/74363-so-you-bought-unweaned-baby.html <--- see post by silver sage (she knows a lot about hand feeding etc). Some breeders encourage people to finish hand-feeding (because of the popular myth that it creates a lifelong bond, but that is false. I believe it probably was perpetuated because INITIALLY it does SEEM like babies are super bonded to those who feed them, but it's very short term and may or may not last. Additionally, breeders often WANT people to take the birds early because hand feeding is so much work and it is cheaper and less time consuming for them if they can get someone to take the unweaned baby.
This is what I am trying to say ^^^
They all SEEM close to whoever feeds them initially, but that is just how babies are. It doesn't last and they often push away from the person who feeds them (as they would in the wild). It is not worth the risk because any "bond" is somewhat superficial when it comes to this stuff (I'm not saying it cannot last, but it is totally unnecessary because you can do the exact same thing with a weaned baby without the danger). You can just as easily bond with a bird that has been weaned fully by someone who knows what they are doing-- abundance weaning is the correct route, btw. Force weaning is cruel and cause lasting damage.
You will have many years with your bird when cared for properly, but so much can go wrong early on--- so many little errors that people don't even think about can have lasting impacts. My parrot likely acquired scissor-beak due to improper weaning by someone else before I got her. Short-term, any baby is going to attend to the person feeding it, but that is just because it is essential that they be fed. If a breeder is willing to sell an unweaned baby, I would seriously question their ethics.