FrogReaver
The most respectful and polite poster ever
By and large 5e offers few choices at character development. A fighting style here a few spells there etc. The only major choice you have is what class to play and since there often is overlap between what characters the classes can represent you may have a few choices there.
IMO just because someone finds an in game justification to do something or gives one in their background or etc. doesn't mean they have let the character organically develop. If true organic development was the concern then you would see about the same number of Fighter 4/Wizard 1 as you do Fighter 5/Wizard 1 and Fighter 6/Wizard 1 and Fighter 1/Wizard 4 and Fighter 1/Wizard 5. But you don't see anywhere near the same. So you are right that it can be blurred. Someone can find a reason or justification for taking levels in some other class. It generally isn't hard to. But their choices on when they take those levels betrays them as checking on the mechanics before taking the multiclass plunge which means that they aren't just multiclassing organically.
IMO just because someone finds an in game justification to do something or gives one in their background or etc. doesn't mean they have let the character organically develop. If true organic development was the concern then you would see about the same number of Fighter 4/Wizard 1 as you do Fighter 5/Wizard 1 and Fighter 6/Wizard 1 and Fighter 1/Wizard 4 and Fighter 1/Wizard 5. But you don't see anywhere near the same. So you are right that it can be blurred. Someone can find a reason or justification for taking levels in some other class. It generally isn't hard to. But their choices on when they take those levels betrays them as checking on the mechanics before taking the multiclass plunge which means that they aren't just multiclassing organically.
Just to add on that, I think this hard distinction between multiclassing for "in-story, organic reasons" or multiclassing for mechanical reasons can be blurred. As an anecdote:
One of my players had this history that he was a member of a merchant family with shadowy affairs, and he was directly involved on smuggling and handling unpleasant figures on behalf on his family. He started thus as a rogue, quite suitable for the background provided. Because of his family dealings, he also got hand on valuable and exotic merchandise, some of which, he noticed, had mysterious properties. One of such items was a figurine in the form of a man with one goat-leg, that he decided to keep for himself, as he felt like the figure looked after him as a kind of protection token.
Eventually, his family got involved in business with a more shadowy partner, that "played on another league", who eventually betrayed and killed all his kin, setting fire to their headquarters while blocking the exits. He barely managed to survive and escape, got all scarred by the flames, and became a hunted man. At this moment, the figurine reached for him. It promised him protection against persecution, and also power to carry out his vengeance, in exchange for his soul. He went for it.
By multiclassing into warlock, the player was clearly looking for game-mechanical benefits. Mask of many faces, devil sight, access to the spell darkness. But the "in-story character" was looking for pretty much the same "in-story benefits". He was hunted, a known face by his persecutors, and even a fairly distinguishable individual by the scars, had his nemesis decided to employ bounty hunters, so in dire need of the disguise offered by the figurine. He was driven by vengeance, and wanted power to exact it, which the figurine was eager to provide. Heck, when I think about it, aren't most choices (not just multiclassing) done at character development a mix of in-story and game-mechanic reasons? Not hard to guess he chose to become an assassin as his rogue archetype, again for reasons on both sides.