I just started a new [url=http://www.bungie.net/en/Forum/Post/81778807/0/0/1]topic[/url] on this, but here's the short version. If you could both buy and sell materials from the faction vendors, you'd be able to trade Crucible marks for Vanguard marks (or vice-versa) at a 10:1 rate, exchange one material for another at a 5:1 rate, and spend marks to gain reputation at an effective rate of 40 marks per 25 rep. None of those things were possible before the patch, and it seems likely that one or more of them was a balance concern for Bungie. [EDIT: Beyond Evolution is right; converting marks to marks is 5:1 just like converting mats to mats.]