Google Enforcing Product Identifier Requirement
Google first began enforcing its unique product identifier requirement in 2013 but it is now kicking off a new round in the next few weeks.
Those that adopted the measure have experienced a greater number of user exposure opportunities on the whole, while those that failed to adopt the requirements have been demoted - so if you're an e-commerce-focused advertiser, take digital heed of Google's announcement.
Google's demand has resulted (in my opinion) in a far richer results page which, of course, put merchants in greater competition across a wider range of products. The requirement, without question, has driven bid prices higher but it matters little to advertisers who are benefiting from additional impressions and conversion opportunities.
Google indicated it will soon start to disapprove products with incorrect data (GTIN, MPN and brand) and demote products that incorrectly use the identifier_exists attribute. Make sure you're in compliance by checking out Google's information on UPI requirements and make sure you're using the latest product feed specification in general.