It's green (the right thing to do and good PR), easy to monetize if critical mass can be achieved, provides more user data, could potentially draw more people to Google+ if that's leveraged for the social networking aspect of the ridesharing (verifying driver identity, etc), and would be a way to stay ahead of the competition (especially now that Apple is pulling away from Google Maps on iPhones). The time is right for this technology (gas prices, ubiquitous GPS, ubiquitous social networking)... just need to get it out there.
In the longterm, this would also fit in well with the self-driving cars initiative. Hope someone within the company is already working on this!
Note: I do not work or invest in any company that works on these types of products. I've been watching it closely since coming up with the idea independently several years ago and would just like to see it take off. (I'd use it!)