I'll admit, this discussion came about because my build violates it.
For example, the first build that comes to mind that the level 20 rule seems designed to prevent is the fully epic Divine Champion. The PrC can normally get an extra 5 epic feats if you start the class at 21 (or 20). The level 20 rule makes it so only 7 levels of DC can be epic, reducing the bonus epic feats to 4. That being the case, wouldn't it be better to re-balance the class itself?
Base classes are also affected by the level 20 rule. I'd be willing to bet that the Monk class was largely considered with the numerous power builds that take a single level just to get the Wis AC bonus. Bard/Sorcerer RDDs as well with the reintroduction of the RDD. Of course, the 3 level part takes care of this, but without the 20 rule, it'd be possible to be a level 1 monk/ninja or sorcerer/bard until level 28. It's times like these I agree with the level 20 rule, but I think it would be better if it was limited to these specific situations rather than a blanket rule on all builds.