Firstly I'd like to say I think someone being able to give a couple the opportunity to be parents is a lovely thing! I don't mean this in a nasty way.
When a baby's born they say they instantly know who mum is, by the sound of her voice, her smell, heartbeat etc. So taking that into account, is it a bit mean to take that baby after it's born and pass it straight to someone else? One of the first things they say to you when you have a baby is have plenty of skin to skin because you are all the baby really knows.
Surely regardless of whether the surrogate used her own eggs or not, as far as the baby's conserned she is mum and she will be the one the baby wants.
"Cruel" is the wrong word I think but it just made me wonder.