I would like to think I could get by with LR5 only?
Some can, some can't - it really depends on what kinds of "effects" and/or special processing you want to have in your photos. I mean, for basic tone & color etc., Lr5 has all you need. But if you want to run LucisArt, or take advantage of content-aware fill, or do things with layers..., you need PS...
Why not try it and see?
What Rob said. It all depends on what you want to do with the software.
Same for camera gear. People often ask which is the best camera to buy. The answer is always the same: it depends on what you want to do with it (not to mention your knowledge and skill level).
John Waller wrote:
(not to mention your knowledge and skill level).
Good point. I'd venture a guess that lots of people buy Photoshop but rarely (if ever) use it, because it's not an easy program to learn...
If you consider yourself primarily a photographer, sticking to Lightroom only will probably improve your work. The problem with Photoshop is that it becomes all too tempting to think "I'll fix that later" instead of getting the shot right to begin with.
If you take a look over in the PS forum you'll see lots of threads like "how do I remove these reflections", "how do I clean up this shot", or the all-out "how do I fix this picture". In all cases, almost without exception, the answer is take ten minutes extra and do it right the first time...Photoshop encourages laziness and bad habits, by its very nature.
Of course if you really need Photoshop there's no alternative.
(edit: I should add that I say this as a Photoshop user since v 7.0).