As others have pointed out, there are things that waze does better than google, such as the real time updates of map errors and traffic reports. Google maps has to wait for the traffic to start backing up, and that can occasionally screw you. Waze seems to sometimes be able to warn you before the cars really stack up.
If google could integrate some of these ideas in improving the speed of real time updates, it could drastically improve the usefulness.
Also, the rerouting functionality of google maps is pretty limited. There are very few routes it will tell you to choose. You can often figure others out by looking at the maps manually, but this is hard if you are by yourself.
If they can integrate some of the better rerouting and real time update tech from Waze, I could see it being a significant improvement to the overall experience. Would certainly keep them well ahead of the race. Sure, they're already drubbing everyone else, but in this business you need to kick them when they're down.