Jones can do a specific job in midfield against teams which require it - that is a Ball Winning, Defensive midfielder who will get about, harrass and press and support the back four.
It is pretty rare that we require this type of player nowadays, but if facing a team such as Chelsea, who have 3 very talent AMs and play a very narrow formation, having someone in the mix there to disrupt them can prove invaluable.
On the whole I think its obvious Jones will end up as a CB, but I maintain (and this goes for all players) that playing in other positions can greatly help a players development and understanding of their own position. If you know your own role, but also know the roles of the players around you inside out then it is surely a big advantage.
Also I think playing in midfield will help develop some of Jones' qualities like passing and positioning, in general.
But he cant half cross a ball...it does seem a waste to have him in a central position when he has proven his crossing to be such a useful tool.