the reason there shouldn't be any standing water is so you don't get the stagnant, stinky water referred to above. that said, I also had the idea of building a bog with one area that would be NOT moving... because I wanted a place where dragon flies could lay their eggs, and I had heard they laid their eggs in stagnant water.
But all in all, it doesn't sound like a good idea to me. We work so hard to keep our pond water clean and clear and DISEASE FREE, it seems to me that stagnant water would just be a breeding place for something nasty that could get into the pond and overwhelm the fish.
Now, if you were to put that in an entirely seperate area, where water wouldn't run off into the main pond, perhaps you could try it. Plant roots do filter out muck and yuck, so perhaps it would maintain itself cleanly.