I think what Johhny is trying to say is you can often live without Lightroom if you have Photoshop and Bridge, but seldom the other way around. His point about not being able to substitute Photoshop is also a very good (and valid) one. Anything, and I do mean anything, of any visual note that's been printed or displayed anywhere in the world right now (magazines, billboards, web advertisments, anything with someone's money involved) would almost certainly have been through Photoshop at some stage of it's life. If you make digital images, it's a bit daft NOT having a copy of Photoshop.
I'm also struggling to think of an area of industry other than event photography where Photoshop isn't used to finalize or just generally retouch the file either. If you're whittling down hundreds or thousands of images to a few marketable ones, you're likely using PS to finish them. The only time I can think not (though granted I don't know about
every area, I'm happy to accept examples!) would be where you have a very high volume of imagery in, and a very high volume of imagery out. Which even in the case of say, fashion lookbooks, all that is still finalized in PS. I know of photographers who only use LR when shooting a test, but as soon as any of their images is being used for something it's usually straight off to the retouch house, and they'll almost certainly still have a copy of PS 'just in case'.
The reason I'm not keen on the Aperture comparison shown above is this. All they've done with that file is basic clean-up, and a b&w conversion. Relatively simple procedures, obviously do-able in Aperture, LR or PS. Now say I wan't to do something a little more advanced, liquify some features, shift some hair around - Oh look, I'm back in Photoshop. Why didn't I just start there from the beginning and have my entire file progress in one nice organised layer stack? Means if the client ever wants something changing I'm not having to remember which program changed which element for one. It's just neater keeping it all in PS.
I use Lightroom as my sort of Bridge now but with a few more features. I've still not used it on a job as a tethering program as every one is too used to Capture 1 (and it supports everything unlike LR) but it seems to have nice enough functionality for it. That said, I could still always drop it from my life and get on without it. With regards to what to process where, beyond the basic RAW conversion (and unless you're dealing with hundreds of images from the same event) I say stick to PS for everything. In for a penny in for a pound, IMHO