I think possibly the last poster sums it up for me, sticking with what I've got for the time being and finding out what the camera is capable of seems like the way to go, well, at least for the time being !
Original question '
Are Primes all they are cracked up to be' is well answered; yes and no, it all depend on circumstance whether its useful to what you want to do..
Now onto next question...'
Should I buy a prime? Would I get any benefit from one?'
There's a lot to be said for sticking with what you have got and learning to get the most out of it, but at the same time, there's a lot to be said for trying new things, and getting alternative references.
You say that the kit 18-140 'seems' to work well for you, and chances are it'll do 90% of everything most folk would ask of it... but with no other reference, would you know when you are asking to much of it? Or when it could offer more, if you knew a bit more?
But, without knowing what a prime can or might do for you, how will you know f it might be worth it, or useful?
Suggestion:-
The Nikon 35mm f1.8 is a cracking little lens; much loved, well regarded and pretty damn cheap. RRP is a tad over £150, but shop around you can get one new for about £120, and they can command almost that 2nd hand; most fetching around the £90 mark.
As mentioned, damn thing keeps giving me oy-dee-uz, and if got one, I'm pretty sure it would make the 18-55 kit I have almost redundant, living on the front of the camera as 'first call' glass, and I'd be zooming with my feet, rather than reaching for another lens, more often than not, and getting back into that bot of diligence and discipline of working with a prime. (Ahrg! I'm talking myself into this aren't I! Dam.. quick, hide my pay-pal password!)
But, FOR YOU.... £100 ish, not a lot of dosh compared to what your camera cost, and as far as higher grade lenses go a real 'steal'... so why not?
Get one - give it a go! -
This will far better inform you of what one can do, than we ever can!
Buy 2nd hand, you'll like as not not loose a penny on one, if YOU don't find it to be all its cracked up to be, or not so useful, and tout back out after a bit of messing. even brand new, 'depreciation' is pretty negligible, it's the cost of an SD card dropped in the grass!
And, its all grist to the mill and more learning; even if you don't find anything 'useful' that a prime can do for you, its still teaching you stuff, and likely to help you appreciate what you are getting from the zoom, as well as likely to help you get better out of it. Just making you move your feet to look for the shot, rather than twiddle a ring, that a prime will beg you to do, is likely to help you 'improve' your photography overall...
What you got to loose? Price of an SD card dropped in the grass...
Its not an 'either or' question, you don't have to stop learning to get the best from your kit 18-140 if you get another lens, its ALL useful learning, whatever the conclusions to the experiment.
Add on Ed: Ie, Gadget Acquisition Syndrome is an occupational hazard... how you cope or not, with that, is not entirely relevant to the subject here, other than to heed the warning it exists. And far as the many gadgets you might acquire in photography are concerned, this could be a 'more' useful one to acquire.