Just out of curiosity...to those who keep talking about "backstory" or 5 to 10 years of experience needed for the first class or explaining how a fighter can suddenly learn how to heal... How are you ok with a Wizard, who learns their spells by intense study, gaining a level in a dungeon and suddenly knowing two more spells into her spellbook that she never had before? ...or a Druid, leveling up and suddenly knowing how to wildshape? ...or a Bard, leveling up and going to college, all while in a dungeon?
I'm a role player, first and foremost. Rarely is any character I play optimized in a way that would make a min-maxer or munchkin player happy. Usually there is a synergy of abilities, and always a flavor or theme, but never at the expense of a backstory. So I understand the willingness to only extend a sense of disbelief so far...however, unless we are truly willing to run a game "hard core" where leveling only happens in a believable way (after the adventure is over, with some kind of training or study or epiphany involved), then we should realize that we have to help suspend our disbelief anyways.
The thing I think that is important, is that backstory shouldn't be more important than evolving story. Sure, a backstory might not contain an explanation for something...but why is a characters now and future bound completely by his past? Shaped by? Yes...but maybe adopting the outlook of requiring the player to present it into the current story...require the player to interact with and involve themselves in the game world in a way that makes their character change. Believe it or not, that is what many players want. They don't want a GM to read their back story and make them play it as their future. They want a GM who will provide them interactions in the game world that allow them to tell a story with their character...influenced by, not bound by, their backstory.
Whether that is multiclassing, or even just in the changes in skills within a single class...there really often isn't enough of a difference to build walls around. Sure...if someone wants to play a Paladin/Assassin, then slap them silly...then mock them until they feel embarrassed for wasting your time. But if someone wants their Sorcerer to multiclass into a Sorcerer/Cleric, is it any harder to believe that they had an epiphany, did some soul searching, and had their prayers answered by Mystra? Is that really any harder to reconcile logically compared to a Sorcerer all of a sudden knowing how to Twin a spell? ...or all of a sudden know two more spells because they killed 10 orcs?