I don't think it's really worth the expense of the 17mm tilt-shift lens for landscapes.

Basically shifting allows you to capture more of your image without recomposing. For example if you wanted to capture more of the top of a building you could shift the lens down (which is effectively shifting the sensor up) and capture more of top of the image. Pointing the camera up would achieve a similar effect, but you would then get converging vertical lines which are undesirable in most architectural photography.

Ok, so you might want to do the same thing with a landscape photo in order to capture more sky. However, given there are no vertical lines in the sky to converge, you're not gaining anything compared to simply tilting the camera up.

As for tilting, you're effectively tilting the plane of focus, and thus DOF (which with a regular lens is always parrallel to the camera). So if you have a wall which is coming towards you, you can tilt the plane of focus to line up better with the wall and thus get more of it in focus.

Again, this isn't as much use in landscape photography where natural features rarely run in straight lines. Plus you have so much depth of field to begin with when using a wide angle lens DOF is rarely a problem. If you're going to use a tilt-shift lens to minimise DOF (for the fake miniature effect) you might as well do that in post.

One thing you can do with a T/S lens is keep the lens still and shift in order to capture a pair of images that will perfecty line up for stiching into a mini panorama, effectively making your camera sensor bigger. But this pales in comparison with what you can do with a multi-shot panorama and VR tripod head.

I'm sure people will point out that there are uses for T/S lenses in landscape (i.e. when you have buildings in shot, or trees you want to keep vertical) but they are relatively few, and IMO not nearly enough to justify the cost of a T/S lens. The TS-E 17 is a nieche lens primarily suited to shooting man made objects / close distances.

T/S lenses are a lot of fun, however unless you really need one they're hugely expensive. You can however make your own using a cheap MF lens and some plumbing supplies. Of course you wont get anything as nice as purpose built T/S, but you're talking about $100 vs. $2000 which is a no brainer if you're doing this for fun!

This is a good post on going the DIY route:

http://blog.cow.mooh.org/2009/07/plungercam-2-cheaper-and-more.html