Faster lenses give you almost always better image quality than slower lenses at the same f-stop. This is true for vignetting, resolution and contrast, distortion and color.
A faster lens also gives you a brighter look through your viewfinder or less noise on your live view display. Anyone who has tried to focus on a star at night knows how annoying a noisy display can be.
Also, fast lenses are usually more expensive and often built better. One extraordinary example for this is Canon's EF 35/1.4 II, which is very durable and is weather sealed. Other brands have well built and sealed lenses, too.
I see no advantage regarding auto focus though, as for one thing no camera I know uses sensors that would benefit from an aperture opened wider than f/2.8 and because modern cameras can focus with as little ambient light as LV -3.
Lastly, if you need or want to take a picture with a wide aperture, you cannot open a f/4 lens to f/1.4, while you can always stop down a f/1.4 to f/4. The fast lens gives you greater flexibility.
There are also reasons to not spend the money on super fast lenses:
They weigh more, cost more and may even require more expensive gear, like a tripod and head that can carry more.
They need more room in your camera bag, leaving less for possibly important stuff like graduated neutral density filters, which often help getting better landscape pictures.
In general, at wide open apertures they are prone to purple fringing and focus shift (this latter problem should noth bother you when landscape shooting).
(Keep in mind, that there are many great and famous landscape pictures that were taken many years ago, using films, cameras, and lenses that today's amateur photographers wouldn't even want to touch.)