[sailfrog]

The rebuttal at ritholtz.com is largely propaganda as well. It was penned by the brokerage firm Themis Trading who has been on a crusade against private exchanges, HFT, reg NMS, and pretty much any change in market structure that could negatively impact their business model. Not only that but the post simply lists properties of the equities markets and implies they are bad. For example:

"-the IOI’s that dark pools send to each other"

If IOI's are bad, why exactly are they bad? The post fails to explain the "why" for any counter argument it presents.

One additional rational not previously mentioned for multiple exchanges is redundancy. Reg NMS allows trading venues to match orders of listed securities from other markets. Because these markets compete as liquidity centers it behooves large market participants to have direct connections to all of them. If the NYSE has technical problems (this happens more often than you may realize, to all the exchanges) trading in NYSE listed securities continues unabated on the other platforms.