Well, sometimes multiclassing is needed to fully use your own abilities.

Take a ID Rager.
They get phantom abilities that depend on slam attack (but don't get slam) while bloodraging.
If they smartly multiclass into Dreamthief Rogue, their attacks count as slam attacks 1/rd.

By multiclassing, they are mechanically and thematically superior (mostly because Paizo dropped ball and never thought about the limitation of having no slam for abilities that rely on it).

What I am not sure about is if they get two emotions or must they choose same emotion for abilities (both ID rager and Dreamthief get access to an emotion aspect for phantoms).

Regardless, multiclassing fixes error in designer creations.