This is a really interesting move. It provides some protection against anti-monopoly actions, as r6y9yd8lar5u1mugbz said, and the fascinating prospect of opening up additional funding avenues and risk-protection as kleinbl00 said. Another interesting dimension is PR – other Alphabet subsidiaries won't benefit from the inherent value attached to the Google brand nearly as much as if they were directly Google products, "Glass, from the company owned by the company who owns Google" just doesn't have the same ring to it. On the other hand if a risky Alphabet subsidiary fails as dramatically as Google+ did, that failure won't reflect on Google and damage its brand nearly as badly. This seems like a win from so many perspectives: Ease of administration, risk avoidance, monopoly defence, additional funding, tax mitigation, brand management. I'm kind of surprised they didn't do this sooner, on reflection.
That's a fine point, I hadn't considered the effects on consumers brand associations; Glass would really benefit from a distancing of Googles' perceived invasions of privacy in particular. It's interesting how we associate traits and values to a name so integrally, that just changing the parent companies name changes how we can perceive the products function. I know I'd instinctively feel more comfortable using the Kinect were it made by Acme Inc. instead of Microsoft. Regarding Google's "Don't Be Evil" slogan, I do wonder how much of Google's fundamental philosophies and business structure will be ported between subsidiaries. It would be amusing and disconcerting to have, say, a military contractor providing cryptographic devices to the NSA as a sister company to Google.