Almost all cool season grass's are interchangeable - fescue, orchardgrass, timothy, brome, wheatgrass, etc. They generally feed much different than legumes - alfalfa, clover, soybeans, peanut hay, etc. Additionally, they are much different from warm season grasses - bermuda, bahia, crabgrass, etc. As long as you stay in a family of grasses, you can interchange easily. When you switch familys of grasses, you can have upset stomachs.
Timothy has long been known as a horseman's grass - for its lack of nutrition. You cannot hardly founder a horse on timothy and it is cheap long stemmed fiber. It is not a valid argument to have any discussion of the quality of any hay type based on species alone. You need to know stage of maturity, harvest conditions, fertility of the soil, etc. A forage test will show quality - everything else is just blowing smoke.