There's nothing wrong with Rijndael. But it hasn't been tested enough for complete confidence, in my opinion. Supposedly NSA takes 7 years to vet a new cipher. If anything, the public cryptographic community should take even longer, given we lack the budgets and accumulated methodologies that NSA can bring to bear.
Testing is the most expensive part of any new cipher effort.
So I think there is a practical basis for at least asking if there is a simple way to combine the AES finalists and take advantage of all the testing that each has already undergone. And, IMHO, it is an interesting theoretical question as well. But it might make sense in some narrow, high value, applications. I agree that Massey and Maurer's proof requires independent keys for each cipher, and have tried meet that requirement in my design. But the fact that Massey and Maurer's proof fails does not mean that the keys must, in fact, be independent for the combined cipher to be secure.