Think the main pro's and cons got covered above. I sit on my cooler with my paco pad on top, and works well, so think you could be real happy sittin on your dry box. Having a good dry bag for a back rest will eventually be key for ya, cause you never get any younger, and it is safer, cause it makes so you couldn't fall over backwords in a flip, or after to many beers!!
Make sure it is not some kind of foam that will fill with water and soak your ass, that would just suck....
We can't always agree, but we can still be civil to each other.