As with any club, the driver head and shaft have equal importance.
Both need to marry together to get the players best results plus look and feel great to the player. As a fitter I think about it quite simply. The head has the majority of impact on what will occur to the ball as it is the point of contact.
The shaft is the timing mechanism that allows the player to deliver the head in the correct space and time... and consistently, the shaft also has an impact on ballistics.
When you marry these together a player sees and feels a good product, and is able to perform a repeatable and timed action for consistent results.