They are valid and tradeable. The system requires
Post# of 96879
No Matter What the Price = Short squeeze and big losses
No Matter What the Price = Pressure throughout the system to make sure that shorts can stand by their commitments
If the Brokerage itself did a poor job of underwriting its customers, then the brokerage goes BK to cover its debts and the risk steps up to the Prime Brokerage that clears their trades. Now we get to Too Big Too Fail.
The problem is that the system isnt perfect and it does break. From our point of view, each individual shareholder retains the value of the shares they bought, but the system may be forced to cover using some sort of E&O or system wide insurance policy or guarantee fund or fraud fund.
Paul