I can't say I really understand the OP's question since the only necessity for playboating is a boat with some nomiinal degree of outfitting - even a backband isn't "necessary". Furthermore, since only Jackson's come with the Happy Feet and there a lot of good playboats from other companies, how could they be "necessary"?
That being said, here's my take:
Happy Feet - If - and this is a big if - you understand the concept (i.e., suck out as much air as possible from the beanbag to form a near vacuum seal), they work reasonably well. But even when used correctly, they do have a bit more movement than foam and may need readjustment every few weeks. AFaust made an interesting observation that I hadn't quite connected before - the Happy Feet did seem to move less in my old 4Fun than in the Allstar. Conclusion - Not a necessity, but not that bad. Foam is better in the long run.
Happy Seat (The airbag that goes in front of the seat to support your legs) - Personally, I love the Happy Seat, with one exception - it makes sponging out a bit harder. Fully adjustable to either just support your legs or really lock you in the boat when you are really throwing it around. I feel way loose in the boat if I don't use mine. While foam knee blocks can kind of get the same feeling, I find the Happy Seat much more comfortable and versatile. Conclusion - Not a necessity, but darn close.
Sweet Cheeks (bean bag that goes on the seat) - Not on the OP's list, but someone mentioned it. From what I have seen a lot of people that don't like these or complain about them moving around, just do not get the concept of how they work. Either they blow air into it or just don't suck all the air out. Here's how it works. With some air in the sweet cheeks, move the beads around til they are where you think you want them, seat in the boat, wiggle a bit until you are sure that everything is comfortable and where it should be, then pretend you are a Hoover vacuum and suck ALL the air out of the seat (you should be able to distinct outlines of the beads. This will create a nice solid seat pad that is custom molded to your butt and should last for weeks or months as long as the sweet cheeks don't have a leak. If you find that seat puts you in an uncomfortable position, blow a little air into the bag and readjust the beads until it is comfortable. Conclusion - Not a necessity but a lot easier than trying to customize your seat with foam and a SurForm.
"It's All Good"