Although technically, MDT can be used as collateral, there are reasons why this is undesirable.
MDT is intended to be used as a recapitalization source for the protocol. This means that the MDT acts as collateral of last resort to absorb losses that exceed the expected losses from the collateral contained in the vaults. If MDT is used as the first instance collateral, i.e., as collateral secured in the vaults, this will deprive it of effectiveness as collateral of last resort.
A negative feedback cycle will occur in the scenario when a fall in the price of MDT will cause the liquidation of the vaults provided by MDT, which will subsequently cause downward pressure on MDT price. While liquidation pricing pressure is a risk applicable to any type of collateral, systemic risk using MDT is potentially higher, since a drop in the price of MDT directly reduces the amount available for the recapitalization of all types of collateral under the MonolithosDAO protocol. The downward spirals in the MDT price also make the MonolithosDAO protocol more vulnerable to management attacks, making the cost of buying enough MDT to vote for an attacker-friendly change more affordable. Besides, if both systemic bad debts and MDT repositories were liquidated at the same time, they would become self-repairing, increasing the risk of a severe fall in the value of MDT and reducing its effectiveness as collateral of last resort.
Finally, there is the circular funding argument. The value of MDT is related to the total amount of MCR in circulation. An additional MCR generated from MDT storages will create a positive feedback cycle concerning the price of MDT, which will create even more MCR from these storages. This behavior increases systemic risk.