Well, I think that it would make perfect sense to phase out using gold and diamond as a currency. However, the entire server would have to be gradually weaned off of it, so that everyone can easily adjust to the new system. Personally, I think that currency should not have a crafting use; it should purely be a medium of trade. Emerald currently has no crafting use; it can only be gotten from two places, which are villagers and smelting it from the ore. Since gold and diamond have tons of crafting uses, this makes both of them bad mediums of trade. But since we have been using gold and diamond for quite some time, the value of emerald would have to be adjusted to something that coincides with both gold and diamond's worth and the easiness of getting emeralds. This value would have to be voted on so that it pleases the entire community, so I prefer not to detail a value for emeralds.
Considering that MTC have a completely different way of being obtained, and their value per credit is significantly less than that of a diamond or a gold ingot, this means that there is no reason to phase out MTC. However, the value would most definitely be adjusted to that of the emerald's value.
But then again, like TPrezzle said, just because emeralds are being added in doesn't mean that diamond cannot be used. In fact, any item can be used as a medium of trade. This is because the only effective economy that Minecraft can manage is a resource-based economy. This means that every trade is done through barter, which is what we have been doing this entire time with diamond and gold. The only true currency we had was MTC, because they cannot be crafted; they can only be obtained. So this means that if you wanted to, you could trade with oak logs, obsidian, or whatever item you wanted, and it would still be an OK medium of trade.
The only thing that we can do as staff is suggest a currency, set a fixed value, and see how the community responds to this change. But I'm not sure how that would work, considering how long gold and diamond has been used to trade.