> Mental arithmetic is a great skill, but I do not think that it should > be taught with an abacus in a classroom setting.
In fact, after becoming accustomed to abacus, students can do it with only fingers, or even simply visualizing the beads in their mind. That is why they use abacus as the first step to mental arithmetic.
Mental arithmetic gives some good confidence in handling numbers to the student (so that they become less prone to errors). But other than that, I am not sure of the use of it.