Why it's good to have all three map services in one place? Simply because there are geographical regions on Earth where the three services are complementary. These complementarities come from many points. The first one is geographically. There are regions where MSN Virtual Earth has good road maps but Google Maps has more accurate image data and with a higher resolution. And also is about the up-dated maps and images. Once they have not the same sources of data (Image courteasy of NASA for MSN Virtual Earth , Data Navteq, Teleatlas for Yahoo! Maps and Google Maps) the images have different resolutions and collection date. In some regions Yahoo! Maps has good city maps with clear specification of one way roads and street names. These regions are in Asia, Africa, South America but also in Europe (Bulgaria, Romania, Serbia, Ukraine) where the need of all these three maps services together is obviously big. For example in Romania Google Maps shows on the map only the main important cities and some localities situated on the main national routes and MSN Virtual Earth shows on the map all main cities and a lot of small villages. But the area covered with high resolution images by Google Maps is significant bigger than the area covered with high resolution images by MSN Virtual Earth and Yahoo! Maps. But Yahoo! Maps has better city road maps for the main cities from Romania : Bucharest and Timisoara. In Argentina MSN Virtual Earth comes with geocoding places and addresses, road maps, satellite imagery and photography and traffic incident and flow information (but you have to be careful - data across the country seems very inconsistent : there are floating parallel roads that lead nowhere and locals notice there are incorrect road names). Google Maps and and Yahoo! Maps have not good road maps, and traffic incident and flow information but they have in some regions of Argentina better satellite imagery and photography.
The second complementarity comes from the additional tools they have and how they implemented them. Getting directions, finding addresses, overall speed of loading programs, overall intuitiveness are some of the features where these three programs are complementary. And in the latest service of these software "Reverse Geocoding" service ( which is finding an associated textual location such as a street address, from geographic coordinates ) there are also important differences. In some regions you can get only the county names for Google Maps and full address at a level of street name and number for MSN Virtual Earth and in other regions you can get nothing from MSN Virtual Earth but something from Google Maps. Also there are differences in the accuracy of data. When you have all these three services together you can have the opportunity to compare them to see what and where you may find better information.
A sample of the implementation of all these three services in one page exists at
www.satelitimobiliare.ro which is a real estate site. Maybe you may find it interesting.
All together are quite useful BUT when you have a good internet connection (if you are lost, didn't get a map of where you were going ahead of time, and don't have an internet connection then all of these services are useless)
