For me, the biggest difference between Photoshop CSx and Elements is workflow -- the ability to automate my work using Actions and Droplets. Elements will do a lot -- probably most of what an advanced amateur photographer needs. But for daily professional use, on deadline, doing repetitive production tasks, being able to customize and automate the application is a huge benefit. For example, if I had to manually go through every step in a simple Save As... command, it would seriously slow down my workflow. So instead I have several buttons for Save As a TIFF file, save as a JPEG file, save as a 16-bit PSD file, saved in various sizes in various permanent folders on my desktop. Click, click, click, I'm done. It's easy to automate profile conversions, local contrast enhancement, capture sharpening, various saves, the creation of multiple adjustment layers, etc.
(Now I need to figure out the Mac Folder Automations, so when the photos are saved to a folder, they are automatically copied to the server, emailed to the client, etc. But that's another project.)