(FORTUNE Magazine) – Some CEOs like it really lonely at the top. An estimated 15% of those who head FORTUNE 500 companies cling to all three top jobs -- chairman, chief executive officer, and president (see table for a sampling). The tactic often leaves aspirants below them uncertain of their futures and can make just about inevitable internal politicking harmful to the company. ''It's the Idi Amin phenomenon,'' argues Donald Hambrick, a professor at Columbia University's business school, referring to the former Ugandan President who awarded himself a dozen top-brass positions. ''The holding of all three titles tends to be a sign of power accumulation and power hoarding.'' Is the answer to designate an heir? Yes, says Carl Jacobson of the MAC Group, a Boston consulting firm: ''The most important thing to avoid in a succession is having a destructive horse race. You've got a bunch of people running for office, and they're going to make short-term decisions.'' But Jeffrey Sonnenfeld, professor at Emory University's business school, cautions against a CEO rushing to name a successor. By keeping open the president's slot, he says, a CEO gains the flexibility to put a number of executives through trial runs at different jobs, allowing him to size them up. That's why Emerson Kampen, 63, keeps the three top titles at Great Lakes Chemical. Asserts he: ''When you dole out titles like president, you automatically exclude everyone else. I think there's a lot to be said for competition.'' Jack F. Reichert, 60, chairman, CEO, and president of Brunswick, a maker of boats and other sports equipment, argues that the arrangement makes sense for companies like his that are decentralized and have strong division heads running daily operations and reporting directly to the CEO. Reichert, who served five years as president before becoming CEO in 1982, says presidents may become frustrated if they remain No. 2 for an extended period. Reichert says he was, adding, ''I think there can be conflicts between the CEO and president over a long period of time.'' A drawn-out term as No. 2, says one who's been there, is ''akin to being Vice President of the United States'' - A.J.M.

CHART: NOT AVAILABLE CREDIT: NO CREDIT CAPTION: CEOs WHO HOG THE TOP THREE TITLES Brunswick Chairman, CEO, and President Reichert