i dont think there is any drawback to creating your live account on the original xbox and porting it over to the 360. MS has said that if you have a subscription then you basically already have the gold version.
maybe when you convert your account over to the 360 your live account is updated with abilities and features that you will be able to access. maybe its easier to add/update your service then it is to take away features or making an update that limits your live account. thats why when you already have an account on xbox1 and port it to the 360 maybe the live servers will then have two references to your account features one for the xbox1 and the other when you log in from a 360. this is all speculation on my part but it makes sense to why you cant use a 360 created live account on a xbox1.
