Many days now I run with a friend at 5am 10 miler then run 3-4 miles later at 5pm with my son and his jr. We could and have debated the 1 per day vs 2 per day runs but to me it all boils down to physical and mental enjoyment. I'd say if you can not find a compelling reason to do it do not. ALL regularly did speed work sessions with the club in the buildup to their better races. Rich Hanna, one of our top 100kmrunners and twice national champion regularly did speed work.
Training is specific.
50ks if the goal was a trail race. Racing shorter events even 10ks on the roads will help your overall running performance. They are doing quality workouts AND doing volume training. Shawn McDonald I will echo many of George's comments, in saying that doing some form of quality workout once a week can be of great benefit in preparing for ultras. Just like building up in mileage, start with a small amount of quality and add a bit each week.