Where the practice with plastic helps is with accuracy and consistency.  Practice is not about scoring.  For the most part, if you are worried about score in a practice block, then you aren't really practicing.  On many house conditions, it is difficult to work on certain components of the game with modern equipment because it may not get the necessary distance to even hit the mark you are working on trying to hit.    

Practicing with plastic is not about trying to rev the snot out of the ball.  

Bottom line, it works for some and then there are others that might not derive the same benefit.  But most will derive the benefit from a polyester spare ball, which makes it worth the $50 or so that it will cost.