I see LR as a workflow tool and PS as a proper image manipulation tool.
LR has some tools that I don't
think ACR has - namely merging to HDR/Panoramas,
ACR comes with Photoshop, so you have these options in Photoshop if you need them local adjustments (with a brush) - including a really good auto mask -
ACR has an Adjustment Brush and the storage of all your edits in a "History" view that you can step back and forward through. You can make virtual copies of images to work on (colour vs b&w for example) without duplicating the image file.
You can make Snapshots in ACR anytime you want, and easily flip between them If you find a group of settings you like, you can save them as a preset.
You can save Presets in ACR ACR users please correct me if I'm wrong here!
Well you did ask.
As well as the above though, LR really shines as a workflow tool.
- Import your photos - adding © data, converting to dng (if that's your thing), make duplicate copies to a backup location, organise into your folder of choice.
- Reviewing your images - Using the grid view,
You can do this in Adobe Bridge or the (even better) survey mode to whittle down your shots to a select "best" few.
- Editing your images (covered above)
- Tagging your images in a variety of ways - colour, star ratings, keywords etc
You can rate and keyword in Adobe Bridge so you can find the images you're after quickly and easiy. Also Collections which allow groups of images to be collected together.
I think you can make Collections in Bridge, though I never have
- Help with Presenting your work - using the various modules (book, web, slideshow)
You can do a slideshow in Adobe Bridge or the Export/publish function to put images on the Web (Flickr) or the Print module if you print at home.
Photoshop is best for (as said above) pixel editing. Want to remove a head or composite a person on your sandy beach? PS is the tool for the job. Combined with Camera Raw, it will do all the editing LR can and more, but it can't manage workflow. Not at all.