I think that the only way it can be useful is if you have someone present who understands the mechanics of the casting. You need to simulate current flow by laying the line out either hard on your right or left side as if it were taken there by current. Then from that full left or right line position you can simulate the sweep - rotation into D loop and forward cast.
Without the left or right starting position you are simply going to be simulating basic roll casts. I have a grass leader that I have used for the introduction of mechanics on my lawn prior to taking people out on the river. I believe it was / is, helpful in that the student has at least a basic understanding of the various parts of the basic casts. Dry training is useful but not for much more than that basic familiarization with motions and the associated nomenclature with those motions.
A few points I believe are important: you do not to make the 'grass leader' tapered but make it with heavy enough mono so that the cut tags will provide maximum drag and resistance., I'm assuming you have looked up the construction of them. Clip the bend from a reasonably large fly and tie it to the end of the leader. You will not need very much power to execute basic casts with the leader on a lawn so don't over power it.
Soon as you understand basic motions and line positioning go to water for practice. Always clip the hook point from flies until you are sure you will not be hitting yourself. If possible find someone to teach you. If a person has what it takes to adapt to 2 hand casting I see them preforming fishable casts within 15 to 20 minutes generally once on the water. Having an instructor will help you greatly even if it's a friend who doesn't know everything.
I realize that some of what I've written may not translate clearly, for instance the part about positioning the line hard left or right.......... This is necessary in order to demonstrate how that line will end up on your downstream side when you are on current flows. The part about finding someone to instruct / help is perhaps the best course forward for the majority of new casters.
I'll leave you go with one good tip. Regardless of whether you are out for the first day or your 20th day on a river or large creek Always be aware of where the fly is located prior to the attempt at delivering the forward cast. A great many people have hooked themselves when spey casting. Some just in clothing or waders and others more seriously so if nothing else......... Always know where the fly is at before you cast.