I think there was enough extra footage that an acceptable theatrical release could have been created. According to David Alexander on pages 225-226 of his book, Star Trek Creator, Roddenberry wrote this to Jeff Hunter (on April 5, 1965) in an attempt to lure him back:
"You will recall I mentioned to you during shooting that I felt there were things more important to both of us than budget. One result of this is we have an enormous investment in a project which can now be recouped in only one of two ways: (1) expansion of current footage via stock and long cutting into an 'acceptable' motion picture, or (2) one day or two of shooting an additional action opening which can result in a fast, tightly cut, exciting film release. Again, the second choice seems best for all reputations involved."
Clearly, Roddenberry felt that it would have been better to shoot additional footage, but he acknowledged that there was enough filmed material to make something work.