[throwawaay]

While it's true that increased competition among market makers have driven spreads down, decimalization was also a factor. For example:

  http://www.sec.gov/news/testimony/052401tslu.htm

"For example, OEA estimates that, from December 2000 to March 2001, quotation spreads in securities listed on the New York Stock Exchange ("NYSE") narrowed an average of 37%, and effective spreads narrowed 15%. An even more dramatic reduction in quotation spreads was observed in Nasdaq securities, with spreads narrowing an average of 50% following decimalization, and effective spreads narrowing almost as much."

Technically, that's an example of a rule that was passed to eliminate the middlemen's profits.