It's this combination of features that honks up the challenge level of a game balanced for a single character, making the play experience less fun. And yes, I argue that the play experience is made less fun by allowing people to semi-simultaneously play two or three or eight or more characters like one ubercharacter, even if some individual player insists that it's "more fun" for them personally if they can play that way.
I've also heard some players say they "need" multiple characters. "Need?" I think they really mean to say they "want" to have a bunch of alts. Anyone who believes that every player of a game -- not just themselves -- truly needs multiple characters per server ought to provide facts and reasoning to justify that belief, and in particular should show how all players actually "need" this capability, as opposed to the individual gamer simply "wanting" it.
In the meantime, here are some perfectly workable alternatives to allowing MCS.
1. Play one character at a time. When you've fully experienced the game through one character class, get rid of that character and start a new one. Why is this a problem?The point to all this is not to be "mean"; it's not unfairly targeting "altaholics"; it's not to be inexplicably looking for ways to make MMORPGs less fun. In fact, the only reason I even express this position (which I know perfectly well that some people are going to claim to perceive as some kind of personal attack on "their" favored gameplay) is because I want these MMORPG things to be more fun, not less. And I'm persuaded that being able to use multiple characters on one server when the challenge level of that game was balanced for a single character makes a game less fun.
2. Start different characters on different servers. If your intention (as I've heard some gamers claim) really is to enjoy different ways of experiencing the game as different character classes, you can do that on different servers. If you're going to claim you can't, then you'll need to explain why you don't just want to be able to share resources to make an ubercharacter.
3. Buy additional accounts. If you're determined to play eight characters like one ubercharacter in a game balanced for single characters, then you should be willing to pay for the privilege of not playing the game as it was designed to be played.
4. Disallow characters owned by the same account from exchanging items. Although players could still use third parties to swap items, this would reducing muling. And while reducing muling wouldn't completely solve the "ubercharacter" problem, it would help.
5. Find a game that's actually been designed to be balanced for playing as multiple characters on a server, rather than trying to push that capability on a game whose challenges are balanced for a single character.
It's easy to look at these games in terms of what we personally enjoy. I do that sometimes myself when I put my gamer hat on. But it's not how a professional game designer ought to work, which is I try to emulate when I put my wannabe game designer hat on. If we're going to talk about what's best for a game overall, then we need to judge game features by whether they maximize fun for everyone who plays the game.
On that basis, allowing multiple characters per server for a game whose challenge level is balanced for single characters looks like a losing design, regardless of how passionately some self-focused gamers will demand that capability.