However, I think part of the reason the rule is in place is because even one level of certain classes gives you handy benefits.
Shadowdancer=HiPS
Monk=Lots. All I gotta say.
Barbarian=Rage, faster movement
Wizard/Sorcerer=Familiar, ability to use mage gear, scrolls, etc, without needing UMD.
The rule seems to have been put in place to prevent powerbuilding, but it does affect the casual gamer as well. Sometimes, rules cannot be made to satisfy everyone, all the time.