There is nothing wrong with SUP if you know how to surf and know your surf manners. The problem is generally with beginners on any kind of board at crowded breaks, those who sit inside on big days and are constantly getting cleaned up by the sets so they are in the way and the greedy. I know people who ride a SUP far better than many on conventional boards short or long.
I respectfully disagree. In my experience, SUPers are the ones who don't abide by proper surfing etiquette. When its decent but inconsistent, they sit outside and take the occasional rogue wave and then have enough time to paddle back out and get in front of you again. It has nothing to do with being a "beginner."
As for me, they get one chance to show proper etiquette, if they aren't don't comply, I won't either. The same goes for longboarders.