It's worth it for a few reasons...
- home automation has been struggling along...quirky, expensive, not quite there. Yet. Nest is one of the few that's made it without turning into x11 crap from china. People are far more likely to allow home.google.com to automate their house than xyzautomagic5567.ru
- metadata is valuable. Even if it's not perfect it's still far better than none for ... so many people. Look at google's cross-platform information usage. Google knows you're married, you google christian dating and 3 weeks later herpes medicine...the following week you get adds for divorce lawyers :) I'm exaggerating a bit but if google knows your home why not display adds for seamless around dinner time? If you set an away for 2 weeks...how about house monitoring / security services?
- The reputation of the company and the inventiveness of the execs has value too. Who else could make a THERMOSTAT cool? Seriously.
- one more way google (and everyone else) can laugh at scada exploits ... or /TinHatOn/ allow the gov't to take over our houses
- on a larger scale contracting with power companies for things like optional temp adjustment on high demand days in return for reduced charges (i.e. raise AC by 2 degrees when it's 105 out and in return your days power costs 20% less...and the power co avoids having to buy expensive power from out of state or go to brown-out conditions)
etc. etc. etc.
There are many reasons if you take a long term view. Looking at just the thing on the wall? Well they still managed to sell an audrino and temp sensor for $250. That's worth something :)