How beneficial is a QS IN the role of a site manager?

A QS in the position of a Site Manager is of great benefit.  They have a good general knowledge of all disciplines on site, and are extremely astute contractually, and commercially.  This can ensure that cost implications and contractual responsibilities are not overlooked on site.  The general aim of this being to increase Project Margin, at the end of the day.

too much,that is because when the site manager want to take any decision ( specially about cost, variations , claims , payments ) these items belong to the QS supporting documents.