It depends how you like to shoot your landscapes. I love the work of a local landscape photographer who regularly gets published in the likes of derbyshire life/staffordshire life magazines.
His method is to stitch 2-5 shots together shot with his 17-40L at around the 35mm region and from what I see it works beautifully!
IMO super wide lenses do produce great landscape shots but give a totally different effect than you get from the method above. I suppose it also depends on the subject too.
If you'd like a link to see his work let me know