The possible pros for mergers of stock exchanges include: economies of scale (reduced costs of managing the exchanges) which might lower costs for traders, and a larger pool of traders . The possible cons: less competition among exchanges which might result in higher costs for traders, and challenges getting the deal finalized (regulations, nationalistic public outrage etc.)
For a brief history of exchanges take a look at Zweig's WSJ article (2-10-2011)
Exchanges Pushed By a Need For Speed
Among the interesting tidbits disclosed in this article: "By some estimates, nearly 300 stock exchanges came, and mostly went, in the U.S. during the 19th century. Many of them died because the stocks they specialized in boomed and then went bust." Speculation, booms, and the quest for information are nothing new.
Review: Definition of NYSE (from Investopedia):
A stock exchange based in New York City, which is considered the largest equities-based exchange in the world based on total market capitalization of its listed securities. Formerly run as a private organization, the NYSE became a public entity in 2005 following the acquisition of electronic trading exchange Archipelago. The parent company of the New York Stock Exchange is now called NYSE Euronext, following a merger with the European exchange in 2007.
Also known as the "Big Board", the NYSE relied for many years on floor trading only, using the open outcry system. Today, more than half of all NYSE trades are conducted electronically, although floor traders are still used to set pricing and deal in high volume institutional trading.
Investopedia explains New York Stock Exchange - NYSE
The NYSE opens for trading Monday through Friday 9:30a.m. to 4:00p.m. (ET), closing early on rare occasions. The market also shuts down during nine holidays throughout the year."