Theme development for Drupal is too complicated, with too many layers. This is part of the movement to clean up rendering of content in Drupal.
The process layer was only created because we needed a place to flatten complicated data structures such as objects or arrays into strings. In practice, render arrays themselves allow for late rendering via theme function or template file (and can be manipulated in other preprocess functions), and objects can implement __toString methods for printing. There is no longer a need for this whole extra layer of processing before rendering.
Removal of the entire process layer.
User interface changes
Removal of the following functions:
PASSED: [[SimpleTest]]: [MySQL] 57,457 pass(es).
PASSED: [[SimpleTest]]: [MySQL] 56,838 pass(es).
PASSED: [[SimpleTest]]: [MySQL] 57,206 pass(es).