We owned one for a short while (less than 2 years) and towed it with no issues. Honda does not approve the 06 MDX for towing four down. They did the 05 and previous models but quietly withdrew the approval about the time we were buying ours. The dealer assured me they would honor the warranty anyway becuase mechanically there was no difference between the 06 and 05 models. If I remember right there was a rumor that folks were not following instructions, like the 65 mph limit, and frying some transmissions and Honda wanted out of the responibility.
Like I said, we were already set up to tow and did it anyway. I was never comfortable about it and figured that if I burned the tranny, it wouldn't matter what the dealer told me. We sold it and got an Acadia.
Should not matter now as the warranty has expired. I would say do it but get the towing instructions for an 05 and follow them.
2013 Excel Winslow 34RKM
2015 GMC 3500 Denali Duramax 4X4 dually