Isn't it the case that the other coin-swapping protocols are susceptible to this same weakness? This applies generally to double-spends too. The cost of mining a big-enough "attack fork" can be amortized over many concurrent double-spend attacks using the same attack fork.

Also....
Please cite this protocol as TierNolan's, I gave a clearer description but changed nothing https://bitcointalk.org/index.php?topic=193281.msg2224949#msg2224949