3D Sprites are useful for a lot of things. Isometric games and 2.5D games can benefit greatly.
Very many 3D games use billboards / sprites for things like smoke / clouds / fire / explosions / lights / flares / bullets / trees / grass / leaves / imposters etc. they're cheap to render and look 'good' from all angles.
there is a new action in Q3D Master called "Snapshot", it's all the way at the bottom. To "save" alpha transparency you first have to set the scene background alpha/opacity to 0. The snapshot action triggers a render so try to put it at the beginning of your event sheet, before you make any changes to the scene for the next frame. There's then a snapshot URL you can access through an expression in Q3D Master, and pass to the browser objects for saving / displaying, or to a sprite etc.