hmm never heard of this rule. my longest drive was 18 hours coming back from disney land, but I wasn't towing and I won't do that again haha.
I quite often do the 10-11 hour trip from my place to my parents in one go. only stop for washroom breaks. the the wife is with me then we split it into two days as he back can't handle 10 hours in one go.
I will do multiple 10+ hour days myself if I am just traveling to a destination where I am gonng to be camping for a while. basicly stop for washroom, to eat, and to sleep.
but I can see where this rule would be good as you get older.