I don't think it matters whether or not a backup wing is more important. What matters is that, once Marbury is no longer an option, there aren't really very many point guards who have a shot at offering more to the team than the pack of what appear to be mostly interchangeable career backup point guards out there who can be had for the minimum.
On the other hand, there is a lot more variation in talent in SF/SG bench players, so spending more effort on that area has a comparative advantage over working on finding a backup point guard, even if the PG spot was more important to fill.