Lightroom vs photoshop is hammer vs screwdriver. Two radically different tools that kind of sort of almost do similar things. (Both the hammer and screwdriver will set fasteners in place and finish a product, but they do so in different ways.)
Lightroom is a work flow focused program that is best suited if you deal with any volume of photos, and by far the most powerful aspect to it is its keywording and data management flow, while bundled with a non-destructive editing and adjustment tools. In just an hour or two I can easily process thousands of images from a set of sports games, select which ones I like, adjust colour, contrast, etc, label each photo with who is in them, where it was taken, what it is about, etc, and then at the end push that data to publishing outlets.
During that work flow I can still pull the image into a different tool rather seamlessly, photoshop, elements, or some other graphical editor, make my changes there, save, and then jump straight back into the LR workflow.
But the REAL power of Lightroom comes from investing the time and effort into adding solid and unique keywords to each photo that accurately describe its contents, and then going back to find specific photos. Shoot a lot of photos for some group or organization who are having an awards banquet next week, and were just told that 'so and so' is up for a lifetime achievement award? Pop into Lightroom, click your root folder, open the the advanced filter tool: 3+ star images, keyworded with "so and so" or "so and so's company".
Bam! The dozens of photos from the past five years that you've taken of them. ctrl+a, ctrl+shift+e, select the ultra small preset, pick dropbox folder, drum your fingers on your keyboard for a minute, open dropbox, share to email... And now you've just sent all those photos off to the head of the awards committee after five minutes of work.
The next day you open your email, read the three or four file names they selected. Fire up Lightroom again, grab those images, possibly pull into Photoshop for detailed editing, colour balance, fiddle with settings till you're happy, ctrl+a, ctrl+shift+e again, select your highres export preset, select the drop box sub folder you've already linked to the head of the awards committee. While they're exporting and uploading you open up your email again, reply to them to give them the heads of up that photos are on their way (Possibly attaching your invoice), and now you've done with less than an hour or two of work something that could otherwise have taken days or weeks of sorting and planning to achieve.
For me, all photos go into Lightroom libraries. Some photos get passed through photoshop on their journey to being publicly viewed, but the last bit of imaging software to touch them before I'm done processing is still Lightroom.
Canon EOS 7D | EF 28 f/1.8 | EF 85 f/1.8 | EF 70-200 f/4L | EF-S 17-55 | Sigma 150-500