I've posted on this before which I want to explore deeper.
People either regard this rule as a joke or something to be serious about. As a joke, many people simply don't believe in it because (using the sales environment e.g.) it's felt that anybody can succeed at selling. On the serious side, it appears that many managers do believe in it due to inadequate or nonexistent training leaving the rep to his or her device (an example is with telemarketing firms, you're often expected to recite a script verbatim, often with rebuttals, which isn't training).
This rule is real for those who believe in it. Based on my experience, I would say that once you get past superficialities and dig deeper, you'll find that this rule has no solid basis (Googling it will turn up many interpretations which is traceable to Vilfredo Pareto going back over 100 years).
The implications are fantastic once you think about all the ramifications of this "rule."
