While you can use D-76, and many other devs, at 1+0, there are advantages to working with 1+1: you can reach a working temp solution faster with 1+1, you always know how many rolls have gone through 1+1 as with 1+0 and pouring back into the bottle, it's easy to forget, especially in a classroom situation.
What I like best about 1+1, or greater, is highlight control. The more you dilute a developer, the less chance you have of "burning-out" the highlights of the photo.
Diluting developer gives you courser grain, not finer. However, this can make a photo look sharper.