Imagine if you were top lane, you see your jungler taking scuttle. In the next moment, you see him spam pinging you to help him when he barely has any hp, and enemy jungler is at half,your enemy laner is csing and you can't tell if your laner is A. still csing or B. Going to help out his jungler. would you help out and keep yours alive for the cost of a double kill or getting yourself killed?
Really depends on your opposing laner, and opposing mid. If your jungler is running toward you and the one chasing him has delayed gap closes and signs of being on cooldown, and your laner is visible and out of position to make it difficult..... go help.
If the low HP is outright dueling, and subject to either a top/mid joiner, just leave him to die.
A lot of times when I'm jungling though.... YOUR laner or Mid's laner are the ones ganking me in my jungle/river. That rather peeves me off when you stay top and afk farm, create 0 vision and I'm suddenly 1v3 in an area where it's normal to path (New Fog of war is going to make this much much worse)
Most of the time i'll just die too, so it's a bad deal.
It's super situational. One of the most important values that I learned in league is the ability to let people make mistakes and not lose everything trying to save them. You know how on airplanes if they're going down you're supposed to mask up yourself before trying to help others? It's a bit like that. If I can crush my lane and do well myself, I have a chance of carrying. If I get dragged down trying to help people who have gotten themselves into dumb situations being greedy, all of our chances to win go down.
Obviously there are exceptions, and there are games that I've saved people from tilting by going out of my way to save them- But it's on a situation by situation and champ by champ basis. One reason I love xerath so much is that, with a bit of map awareness, I can open up on enemies from the other side of the map and save teammates without endangering myself. 
R 
