I once got a good peek at that memo. From what I recall Jon Povill states something like V'ger's purpose is to merge and that Decker has an increasing need to understand this powerful, machine mind and satisfy its desires. Thus, become part of V'ger is his reward in the end. In arguing his point, Povill states that all of this has been building in the first two acts of the film.
Ah, whut?!
The first two acts do nothing to establish Decker's desire to join with V'ger. Illa (and Illa-probe) maybe, but certainly not with this awesome machine intelligence. This is why the ending doesn't feel like a proper climax. It's a good and visually spectacular ending but, unfortunately, it's not properly driven by the characters as Povill seems to emphasize in the memo.
But Povill's point further illustrates some of the structural flaws that hamper the film--and it's at the writing level. Not one moment in the first two acts do I believe that Decker has any motivation in terms of V'ger. He's overly cautious, and doesn't have a sense of urgency about this particular mission. He even hesitates when Kirk suggests that he "get to know" the Illa-probe. He is more of a cockblock than anything else in the film, and really whiney most of the way through.
There's not even one throwaway line to explain Decker's supposed motivation to comprehend this supreme machine intelligence. Well, maybe the "we all create god in our own image" line that's been cut and added and cut again from the three major cuts of the film.
In his review of TMP, Harlan Ellison was right to point out that the better ending was for Kirk to meld with V'ger. In my estimation, Kirk should at least try to merge with V'ger, only to be shoved aside by Illa in favor of Decker.
Kirk seems more anxious than Decker to get to the "heart of the cloud". Once again, with a little tweaking, this "satisfy its desires" could've been given to Kirk, the supposed hero of the film, then at the last minute V'ger choosing Decker over Kirk for whatever reason, known or unknown to the audience.
In the memo, Povill compares Decker's involvement in V'ger's transfiguration to Close Encounters, stating something like one of our "principal characters" is directly involved just as Richard Drefuss was in the climax of Speilberg's film. Yet, Decker wasn't really one of the principles. He was the guest star in a Star Trek, a leftover from the original Phase II two-hour teleplay. Ellison was right--Kirk's the hero, let him do something heroic, not the guest player!
It's like how in Superman Returns, it's Richard White who comes across as more heroic than Clark Kent/Superman.
The merging of Decker-Illa-V'ger is one of the more spectacular events in a Trek film. Certainly, it is the most epic. However, it seems as if Povill, Gene Roddenberry, Harold Livingston, and, perhaps, Ray Wise overwrought the ending without putting the connective tissues to that conclusion into the first two acts. At least that's what I can only conclude from Povill's memo.