Google limits name changes for security and transparency. With the integration of YouTube, specifically, the company has moved to limit commentary to only those with valid Google Plus accounts. Full names also help with security and administration.
Google and Name Changes
Google has a relatively broad policy on the type of name you can have, as of this publishing. It reserves the right to terminate accounts that appear to use a fake name; however, the company seems to recognize the need for changing a name, such as through a marriage or by adding a nickname. To limit abuse of the name changing option, Google allows only three total name changes in a year for Google Plus accounts.