They tend to compliment each other for multiple reasons: Bing has imagery from a different time period (most of the time newer than GMaps, but some of the time older, which can be useful).

Bing also has the Bird's Eye view, and different aerial imagery in general, which can help with clarification and image artifacting.

So basically, we're using all of our available options. BTW, this is all run of the mill stuff in mil/int circles, but with better resolutions and light/electromagnetic bands.