I am basing it on numbers, you are basing it on pure opinion - I prefer my method, to be honest.
If we do sign a CB then it needs to be at the expense of one of the current ones (in addition to Evans, who I am already assuming will be leaving). Rojo only joined last summer, so it is doubtful that we would ditch him. Smalling has improved such that he deserves his spot in the team, and McNair looks a good prospect who we would be foolish to discard (perhaps a loan would be viable, but I think he would gain more by staying at the club and getting gametime in the cups etc - we will have more games next season so he would be afforded more opportunities I expect).
Therefore Jones is the odd one out. I dont mind Jones, I wont shed any tears if he goes but would prefer him to stay. However I dont think it is practical to be entering a new season with four established CBs (Smalling, Jones, Rojo, NewCB) plus McNair. Its simply excessive. There is also a case of the grass not always being greener - just because a player looks good for another team, doesnt instantly mean that will be an instant success for us. Jones may not be perfect by a long way, but he is a proven PL defender with plenty of room for improvement, we could do a lot worse. There is no guarantee that a foreign signing will actually end up as a big step up over Jones. We have seen highly rated prospects like Rojo, Sakho and Mangala all fail to live up to expectation when coming to the PL, and thats just the tip of the iceberg.
Edit: Just realised we are still in the Carrick thread. If you want to continue this debate then kindly reply in another, more appropriate thread.