Resource sharing, while it always looks wonderful from the player's immediate perspective, is a problem for developers because it makes the game as designed too easy over the long term. And that reduces the overall fun of the game whether players choose to believe it or not.
As for the desire of some players to different characters to explore all the possible variations within a game world, I'm sympathetic to that desire, I really am. As an Explorer-type gamer myself, that's actually how I play most single-player games -- I want to see all the content.
But I have to ask those players: Why do you want ten or more characters available at any moment in time on one server? If your goal truly is to explore a gameworld's content, why do you require having all those characters available at one time, on one server? Wouldn't you be nearly as well-served by having many characters on separate servers, or by running characters serially (one after the other) rather than in parallel? You'd still be able to fully explore the gameworld that way, but without opening the door to massive muling... so is there some reason why that's not acceptable to you?
As a practical compromise, I see value in offering a couple of alts in a class-based game. It's an attempt to strike a balance between allowing players like us to explore multiple ways of playing the game, while still respecting the need to apply appropriate limits to individual characters so that they can't be turned into an ubercharacter for whom game challenges are too easy.
Again: wearing our Player hat, it's easy to just focus on what we personally want in a game. Trying on a Developer hat means having to stop thinking in terms of what we want for "our" character right now, and start thinking in terms of what maximizes fun for all of a game's players over the entire course of that game's lifespan.
Sometimes -- often -- that means limits. I believe good designers try to make those limits appropriate for the game they're building, and that's the question I'm trying to get at here: What number of characters per server per account is appropriate when exploring much of a game's content is desirable but resource sharing among characters can make gameplay too easy?
I make no claims to being a professional game designer myself, but I believe it's fair to say that anyone who thinks it's easy to make a MMORPG that can satisfy thousands of gamers should try it.