Even as publicly held companies around the globe display a renewed urge to merge -- such as the $77.2 billion Exxon Corp. and Mobil Corp. pact -- their shareholders have reason to be wary. More than half of corporate mergers fail to create substantial returns for shareholders, contends Chicago-based A.T. Kearney Inc., the management consulting unit of Electronic Data Systems Corp. It defines shareholder return as tangible benefits that investors receive through dividends and stock price appreciation. Among the reasons for failure is slowness in putting top management in place. In a study of 115 multibillion dollar mergers throughout the world and across all industries between 1993 and 1996, Kearney reports that only 39% of companies set up the management team within the merger's first 100 days. Other findings: only 28% of companies had a clear vision of corporate goals during the merger, only 32% applied active risk management, and only 14% "sufficiently" communicate the new alliance.