The brain is also like 70% fat....so it likes fat in the diet....rebuilds and works better......as kids on seizures (like having 5-6 day), went on a high fat diet (which in today's world 30% is high fat) and whoolaa....most kids were down to 1 seizure a week...if any!
Cell membranes need dietary fat to rebuild, hormones need dietary fat....so any diet not getting fat...reduces cell membrane and hormone effectiveness....not too mention the wrong kind of fats (trans) will just harden up the cellular membranes and cause more health issues.
Fats when mixed with protein aid to slow down digestion....aka control blood sugar and insulin response.
As Robb says above...timing is key too as to how things are used....I'm all for the Protein and fats during the day with veggies (maybe some fruit am), but keep the bulk of carbs post workout where the anabolic role of insulin can be maximized. Plus it all depends on your activity level and what exercise you are doing...whether you need glycogen (which is essential for explosiveness in sports) or can get away with more dietary fat (more aerobic endurance related).