While the processing engine in LR 3 and ACR 6 are essentially the same, that takes care of the Develop module. Lightroom has four other modules as well, and while some of it can be duplicated in Bridge, personally I like the 'all-in-one' aspect of Lightroom. I import my images, keyword, add metadata, sort, select, cull, grade, mark, etc in the Library module. I can also do some basic (relative) processing there. If I want to play with an image I press the D key and pop over to the Develop module. If I want to export images to Flickr (or Smugmug or...) there's a facility for that. I can get plugins for geocoding support (Jeffrey Friedl) or combining images for HDR (Enfuse) and a number of other aspects. If there's processing work to be done that LR can't handle, I can right-click on an image, export to Photoshop as an image or a smart object, do what needs to be done, save it, and an edited version pops up back in my Lightroom library. I can create slideshows, web galleries and I can print from Lightroom.
Now, I got into LR1 before I had a lot of experience with PS, so for people who are used to going from Bridge to ACR to Photoshop this is a slightly different perspective. Personally I prefer it, but some do not. You get to choose.