Lightroom is Adobe’s solution to all the photographers in the world that said, “OH DEAR GOD!?!?!? 800$ for Photoshop? Like hell I am spending that much on a program!” Thus, Lightroom was born… unfortunately it was born of a crack-whore and a drunk lunatic. If you like programs that are buggy, and consume your RAM like a virus consumes its host, you’ll love Lightroom.
Now, Lightroom does give you all the control of adjusting your photos, just like photoshop in one convenient area. However, in my experience using Lightroom, I would much rather us Photoshop, that way if I need to touch up a photo, or change something completely, I have that option… Easily. Not to mention I don’t have to worry about the whole, import>export thing. One other disadvantage to Lightroom is it’s horrible printing abilities… or lack there of. I know Photoshop has its problems printing, but frankly with Lightroom, every time you hit print, you are rolling dice… dice filler with rats, rats invested with the plague, a plague of incorrect color profiles and bleached out details.
So to answer your question: Why own Lightroom if you have photoshop? Because you can, if you have 50000+ gigs of RAM, display every one of your files on your massive working screen and sort through them that way… or you can buy a mac and use iPhoto or Finder to do that…. and only need 1 gig of RAM.