Before I really begin, there needs to be a definition presented:
Class: A set of abilities and restrictions designed to enforce and limit the character's progression in certain areas. Some people categorize this as niche protection.
There are two reasons why multiclassing is stupid and everyone who does it is a moron:
1: Multiclassing exists and is pervasive in systems with classes is solely due to powergamers. Oh I need another definition:
Powergamer: Person who (whether by accident or purposefully) ruins games by building the "best possible character" and defining that mathematically with system and in no other way, competing with both other players and GMs in a race nobody signed up for or cares about, and if they did sign up and do care about it they are limiting the fun of the game to the winner or one person. These are broken people who feel they don't have power in this world and would be better off in therapy than at a RPG table. See also: all Pathfinder players who aren't either ignorant or miserable.
These powergamers do not care about characterization, immersion, roleplay, really anything beyond feeling power due to this personality disorder. It may be caused by self-hating due to micro-penis, homosexuality, obesity, transsexuality, ugliness, the meta self-hatred, or hundreds of other possibilities. Powergamers, to be frank, can only be gamers after years of therapy and getting over themselves. The gamer's couch is not a therapist's couch. RPG sessions aren't for working out personal issues, regardless of what idiots say.
So I said all that to say powergamers like multiclassing because it allows for them to broaden abilities and mitigate the restrictions of classes by mixing them together.
2: And to a much lesser extent: people don't understand the definition of a class is to have restrictions along with abilities. In essence what someone with a pure heart wants if they seek to multiclass isn't to multiclass but instead has a new idea for a class. But that new idea also requires understanding why restrictions are in place and that they aren't meant to be overcome but rather celebrated for their existence. Nobody should be playing a fighter/mage/thief, they should instead work out what elements of those classes they actually want, and use that to design the class they were actually wanting to play. If "I want it all" is the answer, see #1.
