There are advantages to OTA PBS if your local station has sub-channels. Some local PBS's have 2 or 3 subchannels being broadcast with different programming, like PBS World, PBS Create. My local PBS has a subchannel called Explore. Even if Dish does provide your PBS station in HD, it is doubtful that the subchannels will be included.
On a side note, by 2013 Dish will be required to provide all commercial broadcast channels in a particular market in HD, not just the big four. I don't know if that will include PBS since it is not a commercial station.