And if so, is it worth doing? Do you gain anything from it other than a 100% complete deal on your map?

You get a few genji gloves which set ur players damage limit at 999,999. U also get urself a gold watch i think its called, for taking down mark 64 (which makes it easier to get 5 stars by extended the target time of battles). If you are going to verse him i suggest fully upgrading ur characters first, and definately having a paradigm with 3 sentinals for one of his moves, otherwise ur dead.

but no like cut scene or story extras?

there are some extra cut scenes along the way, after completing some missions if you go to a certain area you will get a small cutscene between a couple of characters... your datalog has some more story points put into it in the analect section.. There are also some comical cutscenes with some missions.

If you ask me though it would have been good to see more story extras though.

hmm, that's what I thought. bummer.

All done! Mission 63 is a good way to get a free gold nugget if you beat the Adamantortoise. You even have a chance for a Platinum Ingot or Trap (catalyst for making the 3rd tier weapons).