I agree (in fact I made an outright rant about how this game has too many forms of currency), but there is one flaw: If they let you trade Mats for Crucible Marks and XP, and also let you buy mats for marks, there will be a subset of people that abuse the system to buy mats, then sell them back for the XP gain. That said, they could address this by eliminating XP gains for material sales (which is part of what I proposed).