Multiclassing and using prestige classes has been discouraged in many ways, to the point that we don't really do it around here anymore.
* You already mentioned that all classes get meaningful abilities all the way to 20. Multiclass, and you get lower-level abilities from your new class. Usually a loss.
* You get a bonus each time you level up in youir favored class - which you choose yourself. For a single classed character this means every level.
* Archetypes allow a fair amount of deviance from the base class, meaning you can achievie many of the same things multiclassing can by taking an archetype that leans in the direction you'd want to multiclass.