It would be interesting if it only lasted a week. A tweet from Luke Smith said that you'll get rep on the faction and vanguard/crucible at the same time when you pledge to a faction. Since there doesn't appear to be a vanguard or crucible allegiance it might just have to "wear-off." OTOH, it would be better if it didn't expire and you had to be with one faction or another once you pledge. Then you'll always be building rep with the vanguard/crucible and a faction.