Instead of writing a list of reasons why not buy Pentax 645D here is one reason to buy it
Seems it is made for landscape photographers in mind - it is excellent.
Body is weather sealed and a few weather sealed lenses are available.
Its rugged, the grip is awesome and a it has a built in digital level.
Matched with the right lens(es) the image quality is fantastic.
If your main interest in landscapes then 645D is a very good option.
In my opinion, for everything else, other than landscape photography, there are better alternatives available.
Short of MF view cameras, I think Pentax 645D can be a landscape photographer's choice tool.
I've recently replaced my D800 with 645D and I am very very excited and have absolutely no buyer's remorse.
Good luck to you with your decision
Cheers!