The most important take-away from Mandelbrot / fractals as it applies to finance should be the realization that fractals can represent better ways of presenting or simulating financial data than brownian motion / random walks / Black-Scholes. As to why this hasn't been accepted more broadly - well, as the article briefly mentions, there are powerful individual incentives for people to continue to play along in the charade.

If this has not been proved EXTREMELY WELL by events in recent history, I don't know when it would be - but whether from LTCM, or more recently seeing so many CDS etc blow up, it is obvious that many "once in a million" probability events exist than are considered in a proper normal distribution.

Mandelbrot was once asked whether he had any particularly successful strategies for dealing with the market. He said, well, I don't discuss those things - because if I was correct, everyone would follow my lead, and the strategies would no longer work; and if I was wrong, people would discredit the thinking behind it!