Yet another point proving that my oAD&D group played Basic disguised as AD&D. (^_^) I don't find the rule hard to understand, but we found it a needless complication.
I suspect many rules in oAD&D--especially in the DMG--were developed to combat a specific trend, which consensus disliked, in the Greyhawk or another Lake Geneva campaign. If you're campaign seldom runs into that trend, the rule isn't worth bothering about.
Another example would be the "only enough XP to gain one level" thing. The few times this actually came up, the DM usually waived it. Had we had a lot of "characters with two classes" abuse, I suspect the waivers would begin to disappear.