When talk of the Volcker rule really heated up, a great deal of uncertainty cropped up and tanked a lot of stocks, including exchange stocks. The general fear is that liquidity and trading volume will be curtailed somehow. Which explains why the CEOs of both the NYSE Euronext and the Nasdaq OMX have come out publicly against the rule. They argue that the rule doesn't really get to the heart of what went wrong in the crisis and that it would be hard to separate proprietary trades from non-proprietary trades at the exchange level.

But a greater fear is the loss of revenue if business evaporates. Reuters notes a Morgan Stanley (MS) estimate that NYSE Euronext's 2010 earnings would be hit 2 percent and Nasdaq OMX's earnings 3 percent, "assuming all U.S. and non-U.S. bank proprietary trading disappeared and flowed directly to the exchange operators' bottom lines." That's not huge. And it doesn't account for the rise of independent shops that would no doubt open if the banks were to exit the field. 

For more:
- here's the article

Related Articles:
Volcker rule already having an effect?
The coming battle
Can you separate prop trading from agency trading?
HFT shops to rise?