Maximizing gnome-terminal-like windows whose width and height increments in
steps cannot be fully maximized to cover the available screen real-estate;
compiz maximizes the window to the largest step size that fits the screen
resulting in a few pixels of background showing through. 

Metacity allows such windows to be maximized to the full real estate available.
Even though the application cannot use any more screen space than it can with
compiz, a maximized application covers everything behind it. Metacity (or
perhaps the app) fills the little strip with the application background color. A
maximized applicating covering every available pixel of space feels better UI wise.

Compiz SHOULD allow applications that can only be resized in steps (e.g.
gnome-terminal) to cover the entire available screen space (sans panels etc.).