Their is a big subtext that he is doing it for the love of Ilea perhaps to join with her in some way... So why does Decker as he says "want this!" so much?
There is also huge subtext that was in the "ST: Phase II" Writers' Bible, and the novelization of ST:TMP - but not able to be squeezed into the movie - that explains how Will Decker and his mother were members of the New Human Movement. Presumably, Mrs Decker fell deeper into this cult after the death of her husband, Matt ("The Doomsday Machine").
The New Humans were a small but growing movement consisting of humans aiming to advance their race by sharing some kind of collective consciousness when near others of their kind. So the thought of merging with V'ger, Ilia and all the knowledge collected by V'ger was tantalizing to Will Decker, and an efficient solution to their problem of saving Earth.
Had the "In Thy Image" telemovie lead to a TV series, or more telemovies, Decker would have returned to the ship to share some of what he'd learned and take up the mantle of Kirk's first officer, or captain if Kirk returned to the Admiralty.
I find it highly doubtful that much of Ilea's conciousness wouild make it through to the new entity.
They'd already discovered that Ilia's loyalty to Decker had broken through - "Not Decker-unit" - and Spock had seen everything digitized by V'ger had been stored "in every detail".