Frank Oz is a skilled master of puppetry. To replace his work in the OT with CGI would be sacrilege (I thought it was bad enough that Lucas replaced his work in TPM, but the puppet itself was pretty awful).

In the OT, Yoda was clearly intended to be a purely philosophical character –a wise mage and guide for Luke. It was not until Lucas made the PT and discovered the near-limitless potential of CGI that he decided to change the character and make him more physical.

This was arguably a change for the worse. These movies have shown, maybe above all else, that Lucas performs best when his creations are subject to physical restrictions.