Fortunately, I did not have a pair of 6v batteries

The OP documented a specific valid situation that possibly presents an advantage of dual 12s. I say possibly because there are twice as many interconnects with the parallel setup, doubling the probability of an interconnect failure. So while on one hand the consequences were less when the failure did occur, there's just as much chance it never would have occurred at all with dual 6s. For me the advantages of the dual 6s clearly outweigh the disadvantages.