My pal has one on his SSV. Its usefull for a couple of things.
1) Fine tuning the wake - he can tweak his wake at different speed to pull that little curl out that appears. So you can get a clean wake anywhere from 15-26mph or so.
2) He also uses it to keep the nose of the boat down when running so he can see.. or to remove porposing(sp?) when it occurs.
Its not critical, but it is a nice to have. In priority for my new Sanger (which will have a wake plate) I put it the order of PP, Tower Speakers, Swivel racks, Wake Plate.