Mega Gyarados really wants Crunch, but Bite does have merit. Rather than be a powerful STAB move, it'll just have to serve as a "coverage for your usual counters/checks" sort of move instead. If it doesn't get Crunch, the standard setup for an offensive DD set might be Waterfall and Earthquake, with either Stone Edge or Bite in the last slot. If you want to beat Salamence, Dragonite, and Kyurem-B more easily, you go with Stone Edge. If you want to have a weapon for Celebi, Slowbro, and Jellicent, you run Bite. You'll have to choose your counters, but it's not like Mega Gyarados is the only sweeper with that problem.
The real perk of Mega Gyarados, as Yamborksi mentioned, is the fact that it can change up its typing (and as a result, some of its checks) and send its bulk through the roof. 95/109/130 defenses with Intimidate on the first switch-in is just...wow. Just to put this into perspective, here's Scarf Thundurus-T's Thunderbolt against an offensive Mega Gyarados.
252 SpA Thundurus-T Thunderbolt vs. 0 HP / 0 SpD Gyarados: 270-318 (81.57 - 96.07%) -- guaranteed 2HKO
If it can avoid Stealth Rock, it has enough bulk to always survive a STAB Thunderbolt from the most powerful Electric-type in the game with no defensive investment whatsoever. Of course, Mega Gyarados has a few other perks. Destroying Rotom-W and Gastrodon with Earthquake and Waterfall, respectively, is a nice little tool, and it maintains a little over 90% of the power of a Life Orb Gyarados without the recoil. However, the bulk increase and typing change is the most interesting part of this Mega form. It'll probably need Crunch to be great, but even without it, Mega Gyarados might turn out to be a pretty cool sweeper.