The End result is as rolli59 stated - Improved regulation therefore mor stable voltage which may improve OC potential. At what point the Nr of Phase hits the diminiting returns is good question.
A 2nd point is that:
The current through individual regulators is decreased which may translate to Longer life of components (regulators run at a lower temp), lower required ratings for components such as the regulator IC, resistors and capacitors. This advantage is somewhat offset by the higher component count (more components, higher probability of individual component failure)