Before HoW, legendaries from a nightfall were good, because it was a chance for good rolls (field scout/counterbalance vanquisher and firefly outlaw crusader) but now all of the faction weapons can be rerolled, and most of them are terrible anyway. If the nightfall-rewarded legendaries were at 365 attack it would give them an edge over the 331 attack vendor weapons and make them better. HOWEVER nightfalls take 10-20 mins (30 for omnigul) and give a massive rep and xp boost which is more than worth it for the time put in. The random reward is just a bonus.