Grass casting is worth it only if you can't get to water.
I learned the basics on casting ponds and was reasonably okay. Then I got to a river and found myself faced with 20 different casting problems that made all my practice pretty much useless. The easiest cast is a downriver load and a plop upriver. Mend once and you should get a good enough drift to fool a fish. That cast (mastered across both sides of the body) is used more than any other.
There's a Fly shop about 100yds from my house and they do grass casting in the lot near us... seems to always be novices (what do I know) and people trying out gear.