I guess "improvement" is in the eye of the beholder, Patrick. I like the new function as the green dot expansion hides the Dock and Top Menu Bar altogether and widens the window to fill the entire screen, hiding the desktop below, while the double-click leaves both as the window expands only until it hits them. The width of the window is also more limited as the window generally tries to hold the same width while expanding to touch the top menu bar and the Dock without obscuring them. I think it is a real improvement over the brute-force green dot.
EDIT: I've been experimenting with it, and Safari holds the same width but expands vertically. Mail expands both ways, but stops and leaves the Top Menu and Dock visible. So the function is variable. I also tried Messages, Calendar, and Activity Monitor and they work like Mail. In any event, it's an improvement over the green dot.