But you aren't one of the players. In fact YOU aren't a player in any of these hypothetical scenarios.
True. This just illustrates that my motive for this debate has nothing to do with what benefits me personally, or even to 'win' a debate on the internet!
My purpose here is to get DMs to understand themselves. By understanding the difference between things that do
mess with either the game world or the game balance and things that don't
mess with either, then they won't feel the need to blanket ban things in a way that has no positive consequences but do have negative ones.
Judging everything as "not rational" simply because it doesn't match your personal preferences simply isn't, well, rational.
Oh, I have plenty of 'personal preferences', and I'm happy to label them as such. I like warrior-types and don't care for wizards. I've playtested the 5E flanking rules and found them wanting. Elves over dwarves. Loads more.
But, as DM, is it okay to impose my preferences for no other reason than the fact that they are my
preferences? No. There has to be a rational reason beyond
my own likes and dislikes just to ride roughshod over the likes and dislikes of my players if what they like doesn't spoil my day.
If something would mess with the internal perceivable reality of my
game world, then I can veto stuff with a clear conscience. No, there are no gods in my world. No, plate armour and greatswords are beyond their technology. There are elves in my world, but no dwarves.
Even then, am I choosing to create a world without dwarves just because Ted likes to play dwarves and I don't like Ted?
But multiclassing cannot mess with your world because 'character class' itself is not a real thing in the game world. Oh sure, you can have approximations that share the name. You can have a group of wilderness warriors that call themselves 'Rangers', but my single class fighter with the outlander background is indistinguishable in-game
to the other 'Rangers'. Creatures in the game cannot know about 'class & level'; all they get is perceivable approximations, like "Djellibaybee The Evoker can cast spells which roast people with balls of magical fire!", but they cannot know whether he is 5th level or 15th, has wizard levels or sorcerer or warlock or even cleric levels, and they sure as
can't tell that he has one level of rogue!
The reason Order of the Stick characters are
aware of game mechanics is because
it is absurd! It is a comedy after all.
What about game play? Game balance is a thing; I wouldn't let one player have a 20th level PC while the rest are 1st level, because part of the social contract of the game is that the DM treats each player fairly and equally. This doesn't mean that every PC must be the same level at all times, have the exact same number of magic items, etc. But the tendency should be toward parity. It would be strange indeed to have all the players start with 5th level PCs except Ted who has to start at 3rd because I don't like Ted dating my sister!
Certain game elements really can impact the game in negative ways, and some of these are personal preferences that matter and that the DM can and should control. Evil PCs, PvP, that kind of thing can have an extremely negative effect on the game play. Although I don't condone the DM taking agency of a players PC from him, this is an out-of-game issue; part of the social contract which can
vary from table to table.
So in my games, no PvP and I discourage evil PCs. But what if ALL the players said they want to cut lose and play evil bastards who rape and pillage and everything that they want? Well, I have a choice: I can indulge them for a campaign, and maybe use the opportunity to illustrate the negative effects of such behaviour in-game, such as realising at the end that the village they've just massacred for the lulz is the very same village where their regular PCs' families lived, and when we switch back then their good PCs have to deal with the senseless massacre. Or I can just say that I don't want to run that kind of campaign, so one of them needs to run it. If one of them did then I'd play a properly
evil PC and make them regret it! *evil laugh*
The strange thing about multiclassing, and the reason I feel okay to keep banging on about it is not mere personal preference. MCing is strange because it cannot be perceived in-game, so that is not a reason to ban it. Having tribes of orcs living so close to a human city may be 'irrational' in terms of believable world-building (as pointed out in PMing's excellent post) but tribes of orcs ARE perceivable in-game!
You can criticise a fantasy novel where tribes of orcs live unbelievably close to human cities, but you cannot criticise the type-face used in the book on the grounds that Times New Roman doesn't exist in the fictional world in which my novels are set!
MC is also strange in that it is not actually allowing anything the DM hasn't already allowed in his game. The DM can choose not to use the 5E flanking rule because if it is in play then it has an effect on the game play, 'Conga Lines of Death' and invalidating some class abilities, etc. But the MC PC has no ability that the DM doesn't already allow in a single class PC.
So the reason
why a blanket ban on MCing is not
because of my personal preference, but because there is no valid objection to MCing! When it comes to a player's own PC, the social contract is that the player makes the choices for his own PC. Not the other players, not the DM.
The DM can object to a choice that a player makes for his PC if that choice messes with the perceptions of the game world, but MCing cannot do that.
The DM can object to a choice that a player makes for his PC if that choice gives that PC something that the DM doesn't allow in his campaign, but if the DM allows rogues and allows fighters then he already allows the abilities of rogues and fighters in his game; it is not rational to say that 1st level fighter abilities aren't allowed when they clearly are allowed because Jake is playing one! It is not rational to say that learning how to use weapons prevents you from learning skills; it's not a rational position.