It's a bit more difficult than that, and yeah, we really do kind of need rules for it. The major point of AD&D multiclassing is that you sacrifice vertical power for horizontal breadth, but you don't sacrifice half of your vertical power in each class. To do so under-powers characters, since what you get from levels 11-20 is more powerful than what you get from levels 1-10. In AD&D, when most of your party was 20th level, you'd be sitting at something like 14th level in both classes. That means you are still a high level character, rather than just two mid-level characters stuck into the same body. There is also an element of advancement at early levels, where switching back and forth between classes means you will suck at too many levels (takes too long to get your extra attack and decent level of spells if you are a fighter/wizard, for example).
The reason I'd like WotC to do it for me is that they are better at balancing than I am. It is true that AD&D multiclass characters were somewhat overpowered at certain levels, and I'd like to see how they manage to balance these issues while still giving us the feel and functionality of the old style multiclassing.