I often see the E-sports pros buying SS on a number of jungle champs such as 
I figure Lee sin is an obvious choice because of wards hops,but for all the other junglers (who get SS) it's to get a little HP & provide some vision.I see it mostly used on junglers who build tanky.I often play support,so I have a habit of warding.
So my main question here is basically the title.
I often see the E-sports pros buying SS on a number of jungle champs
Because their team mates ward.
I usually get it on 
and 
It's a great item on Nunu . He's great for counterjungling and counterganking, which greatly benefit from the additional vision. And he's constantly roaming the map, so he can easily ward areas where his allies doesn't have the opportunity to reach (such as the enemy jungle).
It can let you ward ahead of you as well, so you don't suddenly facecheck the enemy jungler when you are invading.
I think if your getting sweeper ap nid works good with a sightstone
u can keep her squeeshy and use your vission as defence
allows u to hit people from fog of war i know she has traps but there not a replacement for wards
Unless you are in 5's you are only likely to see it on Lee Sin.
Reason being, more vision is great, but in solo-que it is more important you make a large impact on the game performance-wise, not vision-wise.
depends. If your team isn't warding, pick up the slack. If their team is warding heavily, pick up the slack. All items have a gold efficiency level and same goes for procs of your sightstone. Not sure exactly what it is but if you don't think you'll be using that much gold on wards compared to the sightstone, don't get it. Im guessing its around 10 wards? since that would come out to 750g. So your 11th ward would be gold efficient. Not worth in my opinion. Greens and pinks when necessary should suffice.
However, counter jungling champs are almost necessary to take sightstone. Nunu for example.
And the reason why junglers in pros get SS is to setup plays. Most times solo que teams aren't setting up plays like they are. Besides maybe dragon and baron. Trust me, they're so far ahead of meta.
i'll buy sight stoneor level 2 yellow trinket on any jungler if a jungle fiddlesticks is crushing us
Pretty much just Lee and Nunu.
