This reply from Google explains why they use Mercator. http://productforums.google.com/d/msg/maps/A2ygEJ5eG-o/KbZr_... Not sure if it's really satisfactory, but at least there's a reason.

The projection (how the globe is represented in 2D) is still Google's. Technically it's Web Mercator / EPSG:900913, where 900913 is a rather clever choice by Google.