Just to repeat the important parts above, Magento 2 applications should use the official Magento 2 APIs, not Zend directly. Thus developers should not care which we build upon.
But as soon as you are looking what is under the Magento framework, you are probably doing the wrong thing. Your application code should not care. You should be using the "official" APIs provided by Magento framework so we can change the internals without affecting existing customer sites or extensions. By "official", we are going to document which APIs are "supported" APIs (not done very well yet) - to make upgrades more reliable, we are going to keep these APIs as stable as possible, but make changes to the underlying code base to improve performance etc.
Note: I don't look in this area very often - Magento 2 questions are watched on the GitHub issue tracker. We are trying to keep them all there at present.