Let's say I have 900 health (Second Wind)
I get attacked down to 300 health and start recovering my missing health from the rune. That's 4% of 600, plus 6 flat. 30 health. So having this rune gives me effectively 30 extra health if I roam out of range from followup attacks to get its full effect at what is a fairly low amount of health.
930 / 900 is a 3.33% increase. What else could increase my survivability by 3.33%. Hmm, how about 3.33 armor or magic resist. Good thing the other runes in the same slot give much more of that stat than that, especially conditioning, which gives 8% extra health against both types of damage in addition to its % armor/MR increase. This is just during laning phase obviously, otherwise it gets even worse at higher health values where the flat 6 health decreases more and more in value. Even at its best case it's granting an effective 4% health increase if you slip from the narrow clutches of death, but you'd be backing in that case anyway.
Why would I ever want to take second wind over any of the other options in its slot? If I'm against AP in lane, mirror shell is giving me effectively 2% extra health boost at 0.0001% health, and 6%at full heatlh; Iron Skin is the same with 1% and 5% respectively, and conditioning blows them all out of the water if I'm willing to wait it out.
Is it just for extremely low mixed damage poke that for some reason has a 10 second cooldown? I'm just having trouble seeing any remote value in it compared to the others in its slot. When it was in its own slot competing with bonus shields/healing and permanent %health increases, it at the least offered a bit more in terms of decision making. I can see why having it, conditioning AND boneplating could be a bit much, but as it stands, it just doesn't feel remotely redeemable for its slot.
. If an attack takes you from 500 to 580, you get more staying power by taking a single hit and backing off to heal up, then going back in, than by taking extended trades. 20 is being a bit generous that early into the game (needing 350 missing health or around 50% before items get bought), and as said it gets worse than just armor or MR at higher health values.