There's a pretty significant difference in how LR and Photoshop operate and there are some significant
factual advantages to LR (assuming we're talking PS proper, not ACR):
LR processes all data high bit, wide gamut even if the data isn't such. PS doesn't.
LR has an adaptive interpolation, it's a steeples resampling so to speak. You don't have to set anything; it's smarter than PS and knows if you're sampling up or down. PS has five options for interpolation.
LR provides unlimited History steps which remain with the image data (database). Photoshop doesn't and you lose history the second you close the document.
LR provides all processing applied by the user in what Adobe feels is best order. You can move about the controls as often and differently as you wish, LR will process those steps based on best order of edits. PS doesn't. You can if not savvy editing, chase your tail and end up with more data loss depending on the editing order. PS burns the edits as you save the file (yes, layers provide some additional flexibility but outside of Photoshop, if you print the document, or save it outside PS, those edits are burned into the pixels; no free lunch here).
LR doesn't have any need to open the entire document, raw or otherwise into RAM, pixel for pixel. It's a proxy editor meaning, it works with previews it generates. Then when you wish to render edits, it applies your parametric edits (instructions, text) to the source data. Photoshop can't even preview the image until it opens the entire document into memory!
LR is multithreaded. You can select 100 thumbnails in the Grid, apply one or multiple edits, apply an export preset, print and immediately move onto another task in that or another module while LR processes those 100 images. PS can't do anything like this! It has to process the images one at a time and you're freed up to work after it completes that task.
If you soft proof in LR, you can produce output specific edits on a virtual copy (dozens if necessary) and the rendering intent you select is honored when you print that document(s). Photoshop only saves the last settings for printing OR the last saved printing in that document, otherwise you get to reconfigure the Print dialog each and every time.
There's lots of functionality PS provides LR doesn't. Just like a 4 wheel drive Jeep can function in some situations a front wheel drive, sport's car can't and vise versa. Use the right tool for the right job!
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Andrew Rodney
Author: Color Management for Photographers
The Digital Dog
http://www.digitaldog.net
