The reason why everyone wants Slade to make the move to centre is because Ford looks like he could become a really really special player, with Farrell as a more than able deputy- both of whom are more or less Slades' age. Why waste a talent like that on the bench/not even in squad until maybe one day he surpasses Ford (will it ever happen?) when he could make the 12 (13 maybe) spot his own, which is a position in which there is, and for the foreseeable future will be, sparse pickings in terms of actual talent.
Imo, he has looked better at 13 than at 10 for Chiefs (lots of variables involved, ok), the wabblies have done fairly similar things with Toomua, Lealliafano etc., who often play 10 for club, but 12 for country, because they want to put their best players on the pitch.
I could quite easily see a backline of: Ford, Nowell, Tuilagi, Slade, Joseph, Watson at some point in the future.
bring in a guy like Daly at fb, and you would have a team with pretty interchangeable roles at 1st/2nd receiver, you could also take liberties with the bench- not having to pick an extra fly half with Slade and carrying an extra forward or 2 wingers etc.