Bridge + Photoshop ≠ Lightroom
Lightroom is different in a fundamental way: it creates a catalog specifically for images (and now videos), and provides a toolset to manipulate images. It creates a catalog for sorting and finding images in a (supposedly) more efficient way than Bridge. If you work on a project where you create and manage a couple thousand images, Lightroom is (supposedly) a better program for editing & sorting the body of work. The cache and previewing engine of Lightroom should be faster and more stable than Bridge because it's specifically designed to handle images.
[I say "supposedly" because I use Bridge daily as a project tool. I've use Lightroom v3 on a few projects where thousands of images were generated. I'm on the fence with regards to its file management protocol.]
Bridge is a tool that is best used when you are using many Adobe tools (Illustrator, InDesign, Photoshop, AfterEffects, Premiere, etc.) on a project. It's a great tool for sorting project files, finding & opening those files into their respective parent program. Its cache system is still a bit weak-kneed when thousands of images are managed. It generates "hidden" cache files and project files that are critical to its functionality. Lightroom creates a directory for the storage of cache, previews and settings with the idea that it will be faster and more robust (i.e., stable) in generating image previews and managing hundreds of images.
With Lightroom, Adobe also targets photographers who don't want to use Photoshop on every image. The basic toolset allows someone to make global and some local adjustments that, in many cases, provide adequate post-production results.