For me, collateral is locked and cannot be traded, transferred or moved. Thus "liquid" BTS are not backing anything.
However, I could understand if people thought of additional "inertia" coming from collateral for bitEUR, bitCNY, bitGOLD, because they reduce the supply of liquid BTS and thus result in reduced sell-pressure and this way support the "peg" during margin calls.
Whether the marketcap of BTS becomes bigger than the sum of the "market caps" of its bitassets needs to be proven first. Hopefully Metcalfe's law applies here as well :)