I feel the same way with my wife adding fabric softener or natural vinegar to loads. I would expect the vinegar to throw off the ph of the water and make the detergent not work as well as it's supposed to.. but to each there own I guess.
Most fabric softeners contain animal fats.
I have always heard that using it too often is very acidic and bad for plastic and rubber seals on a washing machine. More so front load but also top load.