nospam
photoshop is an extremely capable image editing app but it lacks some of the stuff lightroom has.

Savageduck
Other than the cataloging/library features they are very similar animals even down to using the same RAW processing engine. Once you consider PS comes with Bridge, you just have a slightly different workflow to learn to gain similar benefits from either LR or PS.

Tony Cooper
And vice-versa. LR lacks some things PS has. The better system is having both LR and either CS or Elements.

Savageduck
For most photography oriented folks LR is all that is needed.

Once you add in a need for major cutting & pasting, then PSE or CS6/CC should be added. Also, if you are doing any compositing, PSE won't cut it, then the LR5 + PS CS6/CC workflow option is the way to go.

RAW processing with the version of ACR packaged with PSE is well short of the features found in LR5 and PS CS6/CC.