In my learning to spey cast, I worked the majority of the time on still water. Considering that running water suitable of spey casting is two hours away, the lake 15 minutes from my house meant more casting and less driving.
In teaching spey casting, I like to go over the basics on grass, lift, sweep, anchor placement and Al Buhr's "flip the tip" excersise. I also like to work on overhead casting on grass. The it is off to the lake. Still water lets you stop the cast and examine the line posiiton.
A friend spent some considerable amount of time grass casting. We then met up on the water and casts together. Due to grass casting, his anchor placement was non existent. He was blowing the anchor on every cast and did not notice casting on grass.
Also as mentioned above, on still water you can cast on direction and then either cast with the other hand or do an opposite shoulder cast to reposition the line. And their is no waiting for the line to wash down the current and get tight on the dangle.
My casting transitions easily from still water to moving water. I am a much better casting for all the hours spent on my local lake. Don't let anyone tell you that you can't spey cast on still water............