MACs could be useful ...
If they really are gathering the details of all the MAC addresses in use, as opposed to just those of the access points, there are potential (though very likely pretty small) worries:
How securely will Google keep the data they collect? If you're using MAC filtering on your network, you might not appreciate someone gathering a list of all the MACs that you have, and then putting it in a big data centre somewhere.
While the MAC address wouldn't normally get outside the home network, if you're connecting to someone else's network, whether an office or a coffee shop, they're going to know your MAC address too.
That comes back to the first point; what are they doing with this information? If there's any way that someone else (including law enforcement) can determine that the laptop used a hotspot X on a certain date may be kept at address Y, that's more information than many people may feel happy giving out.