The “shift” capability has traditionally been used for correcting converging verticals, and is a welcome and necessary feature. However, the “tilt” capability is much more useful: it allows the plane of focus to be made more parallel to the film/sensor, increasing the available depth of field dramatically. In essence, you can “cheat” and obtain the depth of field of f/16 at f/2.8! Subject matter can be more or less cooperative of course; we’re talking about planar alignment here, and lots of subjects don’t fall into a plane.
The tilt capability also allows many creative possibilities by forcing portions of the subject out of focus deliberately. Many of the images of Vincent LaForet use this technique in his essay The Cloud is Falling. See other examples in my first impressions of the PC-E 24mm.
My recommendation: tilt-capable lenses are a must-have for evading the diffraction/contrast/sharpness issue with today’s high resolution cameras, not to mention the creative possibilities, correction of converging verticals, etc. See also my 28mm Shift Lenses review.
I also recommend the PC-E 24mm f/3.5D ED (see my first impressions). I expect, if anything, that the 45mm will be as good or better than the PC-E 24mm, because a 45mm lens is generally easier to design than a 24mm lens.
Nikon PC-E Micro-Nikkor 45mm f/2.8D ED