I really wanted to avoid commenting on this argument again, but I feel I have to.
This madness? The idea that having 4 brilliants players for two positions possibly upsetting the balance of the team = madness. Really?
I thought it was a widely-held belief that having a settled defence is the ideal scenario. Ideally you would love to be able to go through a season with the same defence and keeper, they would build up a solid understanding and the team would benefit as a result. The reason you cannot do this is because of injuries and general fatigue. So managers freshen it up by rotating. Too much rotation could lead to an unsettled back-line and be a detriment to the team. Look at De Gea and Lindegaard - Fergie rotated them for a while and neither one was able to establish himself as first choice. DDG only hit some consistent top form when he established himself as first choice. If we went out and signed Cavani tomorrow what would you do if we found ourselves playing City in a title decider? Drop Van Persie, drop Rooney or drop Cavani? How do you think they'll react to that?
What you are suggesting is that we have four equally good players while still having two clear first choice defenders. Can you not imagine the two equally good back-ups getting slightly miffed at having to settle for the odd cameo.
Your example of Ferdinand and Vidic being the first choice is also flawed. Yes, they were indeed first choice, up until last season, but they were exceptional players and the back-ups were promising youngsters who weren't on the same level. We didn't have four players on the same level, we had two world class CBs and three promising youngsters. Instead of Evans, Smalling and Jones, imagine our three backs-ups were Hummels, Pique and Chiellini. How would you keep all five of those happy?
A managers job isn't simply to stockpile the best players, it's to build the best squad - and the two aren't the same thing.
EDIT: And I'll repeat once again, that I agree with you that we should be buying another CB. I don't think Evans, Smalling, Jones and Youth Player is good enough for what we should be aiming for. I just really disagree with you taking issue with someone for suggesting that 'rotation risk' (ie, the risk that rotating players to keep everyone happy can lead to a team becoming unbalanced and lose consistency while also hampering the players' own development) is completely unfounded.
When did I mention heaving 4 great defenders or having the players of the standard you suggested as backup? I said there was no issue if the ONE CENTRE BACK WE ADD is as good as the 3 WE ALREADY HAVE, but somehow it would be better if we added a 4th centre back who is worse than what we already have? How does that make sense? That's what I'm arguing against, the idea that this summer we should seek out and deliberately add a worse defender than those we already have, rejecting better players, rather than someone of similar standing, the argument being that Indi is supppsedly too good to be a squad member, so we should sign someone not as good as him. It's a ridiculous notion.
We need another central defender, we agree on that, so what we should do is not sign players as good as we have, but actively go out and look for lesser players. Do you seriously agree with this premise? We shouldn't sign this guy because he's as good as Smalling and Jones, but we should look to sign a worse player instead, because that's what has been suggested that I disagree with.
If a player of similar standing to Evans, Smalling and Jones is available, wants to come, is happy to fight for his place, is financially viable and won't impact our ability to sign the player we need in other areas, why would we turn down the chance to sign him and actively look for someone not as good? The other guy suggested that if we were to sign a top quality centre half (ie Hummels) we should sell Jones and replace him with a lesser player for the squad. Again, how does that make any sense? You don't actively look for worse players than you have if better players are available, and you certainly don't sell the ones you have to deliberately buy worse ones.
I'm not advocating going out and signing 4 world class defenders, or even saying that's an ideal scenario, I've never once said that. I'm saying that turning down the chance to sign this guy because "he's as good as Smalling and Jones, so signing a worse one would be better" is a silly idea. Don't you agree?
Also, I've never suggested we should rotate or not have a settled backline, but having good players in reserve is better than having less good players in reserve. Surely that much is obvious?
Here's a question. You agree we should sign another centre back, so all things being equal (ie availability, impact on spend on other areas unaffected, the player is happy to come and fight for his place etc), what kind of centre back should we go ideally go for? One who is better than what we have, as good as what we have, or worse than what we have?