I'm curious to know why you feel that Conditioning is a weak rune? Its slow stacking nature is definitely a bit harder to feel than other runes that have a clear on/off state. This would explain it feeling a bit weaker than other defensive runes.
Because this rune is something that all champions have access to it actually wouldn't help tanks more than other classes. It's actually a bit worse for tanks, vs other classes. This is because they end up buying extra Armor/MR so the bonus you gain from Conditioning ends up not being worth as much for tanks, because Armor and MR values have diminishing returns, when compared to other classes who don't purchase as much.