I have long said that most photographers using Lightroom and Photoshop Elements will be able to better work with their images more effectively and more quickly than any but the most proficient users of Photoshop. And they can do it at much less cost. Being able to work quickly and efficiently is important for most nature photographers, including me, because the joy of photographing nature is from photographing nature, not spending more time than needed in front of the computer.
Lightroom does a lot, but when you need layers, it won't work. Having a Photoshop program gives you access to layers. Layers are extremely helpful for adjustments with adjustment layers, for doing cloning to fix problems with an image (such as flare), and to combine images for panoramics or double-processing of RAW. Double-processing of RAW is a very important technique to learn for nature photographers and I discuss it on my website at www.robsheppardphoto.com. This is really easy to do with Lightroom -- simply make a virtual copy of the original file, then process these two images two different ways, one to optimize bright areas, one to optimize dark areas, then send them to Photoshop or Photoshop Elements for combining into one final file.
In the past, this was not so easy in Photoshop Elements because you had no access to layer masks for layers with pixels (though you did get layer masks with adjustment layers). Photoshop Elements 9 changes that. Layer masks can now be used with any and all layers. That is a huge change and really opens up possibilities for things like double processing of raw, two-exposure images, panoramics and more.
While Elements 9 did not get the fill part of content-aware technology, it does have it for the spot-healing brush. You can now brush over a problem and have the program fill that problem area with detail that matches the surroundings (to be honest, this does not always work 100% of the time, but it works enough to make it very worthwhile -- if you try it and find it is not working, change the size of your brush and try again).
There is now very little that Photoshop Elements 9 cannot do compared to Photoshop, at least for photographers, and it is in a more photographer-friendly interface (including such things as Guided Edit that actually guides you through certain processing steps). And at a price of under $100, it is a terrific value (especially compared to Photoshop!).