The Economist magazine has a characteristically lucid—and characteristically opinionated—special section in the issue of 26 Feb 2004 entitled "International Law Firms," and posing the question:
"Is it time to abandon the idea of assembling a truly international law firm?  [Does] commercial law [call for] small local specialists rather than big global generalists?"
Their answer, in a nutshell, is that duelling economic models of partner compensation between the U.S. and the UK militate against a large wave of mega-mergers.

Bonus fact:  Nine of the ten highest-profits-per-partner firms are headquartered in New York.  Let's hear it for the home town!   And the tenth is not in London.