The problem instead is that should the price of MasterCoin fall, the value of all MasterCoin in escrow will not meet the intended value of all issued PlatinumCoin. A similar price mismatch will arise should the USD price of 1oz Platinum rise.
Currency can be configured to automatically dissolve if escrow fund is too low, from the 1.1 spec (this wasn't present in 1.0):
If the currency creator had set the minimum escrow size to 100% the escrow fund would never get into
this situation because it would simply dissolve and pay out to currency stakeholders as soon as the
escrow fund value dropped to parity, with zero or minimal losses.
So... If mastercoins drop in price against gold, your GoldCoins will be automatically converted into mastercoins. Isn't that helpful?
Furthermore, suppose mastercoin drops in value. This triggers automatic conversion of GoldCoins into mastercoin. Now we have (potentially) a lot of people, who preferred gold exposure to Mastercoin exposure, and who know that mastercoin drops in value... Obviously, they'll want to buy gold with their mastercoins ASAP. Which means that a lot of mastercoins will be dumped on exchanges, causing further drop of mastercoin price, possibly causing collapse of other escrow-backed currencies.
So this has a positive feedback effect, where relatively small fluctuations can trigger an avalanche.
This seems to be an inherent property of systems like this: you cannot achieve true stability within a closed system; you can, however, make a trade-off, that is, you have to pay for reduction of volatility, and the price might be increased probability of catastrophic collapse.
MintX is expected to serve as a market maker
By the way, escrow fund mechanism makes it harder for a honest company to do this market making.
Suppose escrow fund is not used. Then a company can back coins it issues with 100% physical platinum. Company's exposure is neutral, so it doesn't need to do anything sophisticated to keep currency alive. It can simply sell platinum and buy back coins. It is inherently stable.
Now suppose that company buys mastercoins instead of platinum. Now company's position isn't neutral: is is long mastercoin and short platinum. It will be very hard to hedge this exposure.
Honestly, I don't see why a honest company would want to start an escrow-backed currency.