This is only an opinion, but I think, after reading the law and information about when I put together the advisory about registering your child, that they want it mostly for a paperwork thing. For example Ricki, I think if your son went on his British passport, but later on decided to assert his right to US citizenship, they would pull the record on him, see he entered on his UK passport when travelling to the US and it might get confusing. Or if they didn't find his trip to the US, they would still have two files for him, one as a non-citizen, and one as a citizen. If any official saw the two files on him, they may spend a load of time chasing it up to find out if it is the same person. Either way, it may take a lot of time to sort out, and I think that is what they are trying to avoid by having people do it early. I think it's not wanting to have two or three files for one person. I think it's also so they can keep a total of how many people could enter the US legally to stay as a citizen at any one time. I don't know if it's arrogance or just wanting to keep the numbers accurate.
I personally feel parents will do what they feel is right for their child in this matter, so please don't think I'm trying to tell anyone what to do about their child's citizenship. I'm really not. I apologise if anyone thinks I'm trying to advocate for one side or another.