If you have a set of 10 childless transactions, of which 9 are normal-fee and 1 is low-fee, and the low fee is above some tiny threshold which is the cost of hashing etc, would you lose the opportunity to earn some pennies that are just lying on the road? You can take 1% of the available fees if are are afraid of being orphaned.
As far as I understand, the low fee transaction will likely be orphaned either by you, as you can inherit not from the low fee transaction but from it's parents, or by someone else. I agree though that the design is very complex.