Hi,
Thought I'd add my 2 cents...
Wash riding is pretty common in marathon kayak races (flatwater). As stated above, it's pretty hard in chop or ocean swells, and hard to hold an ideal position.
But in flat water it's very effective, and if you do coaching or club sessions they'll usually teach/practice wash riding. I did read a study that said it reduces effort (for the same speed) by 17% - 30% depending on position and conditions. But you also learn how to rid yourself of a wash rider

. In lots of races ther'es a pack until the last km and then it's a sprint.
It makes the race about tactics and strategy as well as strength/endurance. The general rule though is not to wash ride soemone in a higher (or any other) class, and officials look out for this.
Often you will get a few paddlers who share the load and plan the race together, makes it interesting and more social. In other cases it's a free-for-all and paddlers occasionally get sent into trees, banks, bouys and bridges!
So I'd see it as natural for it to find its way into SUP flatwater races. Makes the races interesting, makes you think about your tactics, and means the biggest paddler doesn't always win.
All good fun.
Regards,
Darren