Pretty much the only advantage of 1080p is in case the TV set is capable of accepting 1080p24 (24 frames per second) and can correctly convert that to 96 or 120 frames per second (a multiple of 24). That results in smoother motion without any frame loss. But unfortunatly very few TV sets on the market can do this correctly.
Besides this motion/frame loss thing there is no advantage in 1080p vs. 1080i. It's the same resolution. The picture quality simply depends on the amount of compression used. If Dish used the same compression level for PPV as Blu-ray then they could achieve the same PQ if they wanted to.