Let's take two characters.
One is an assassin, specifically, an assassin in the brotherhood of the crimson rose, trained to complete their missions no matter what the circumstances - fight with anything, blend in when needed, hunt their victim as long as necessary. He's been an assassin since he was a child, and he plans to stay one until he get too old or rich enough to retire.
The other started out as a merchant guildsman, then joined organized crime when his business was destroyed by war. He learned Necromancy, and rose to become the classical evil baron. Then one day, he had an epiphany and gave up his former ways and rank to become a champion of good, defending the people from extraplanar horrors. After many years though, he came to realize that the tide of creatures from the lower planes could not be stopped by normal means, and set himself to mastering the planes in order to find a real solution. He's changed paths several times, and may do so again.
Consider which one is more consistant and which is more spread out.
And now let's look at their classes. The first is a Rogue/Monk/Fighter/Ranger/Bard/Invisible Blade/Replacement Killer. The second? Just a plain Wizard.
Probably not entirely unexpected, but I think it shows that the classes are not the same thing as the character, and that "cherry-picking" makes no more or less sense from an in-game PoV than going single-classed.