Fanaelialae
Legend
Don't like this. A 10/10 fighter/wizard shouldn't be able to cast 9th-level spells at all. Should cast spells as a 10th-level wizard (max spell level 5). Want to cast 9th-level spells as a 20th-level wizard? Stay in the wizard class, don't multiclass into something else. That's the price you pay. (Not saying anything is wrong with multiclassing, but it's a choice. You choose to branch out, you aren't as good in your primary class anymore.)
I disagree with this. While I don't think a 10/10 F/W should be as good a fighter as a 20 F or as good a wizard as a 20 W, I do believe that the 10/10 should be as good as a level 20 character overall. In 3e, that definitely was not the case.
The reason for that is that not everyone multiclasses for reasons of munchkinism. The most obvious reason for an open ended multiclass system, similar to that found in 3e, is so that players can mix and match elements in order to achieve a particular character concept that they wouldn't be able to express otherwise.
For example, perhaps a new player is fond of Lieber's writings, and decides to create a character based upon the Grey Mouser. So he takes a few wizard levels, and then a few rogue and fighter levels. He should be able to do so without negating his ability to contribute, because multiclassing that allows you to create a useless character is effectively a design trap. The player is unlikely to enjoy the game as much once he realizes that his stylistic choices have rendered him into an anchor upon the party.
IMO, choosing a level in a different class should be neither better nor worse than choosing to advance in a single class. To do otherwise is to create false choices, and a false choice is arguably worse than having no choice at all. At least if you offer no choice, a newbie isn't be punished for making the wrong choice.