User blog comment:Deshiba/Heal on supports/@comment-1330314-20170323062958

The benefit to Heal isn't simply that it heals, but also that it provides a movement speed buff. If you were really in sync with your partner, you could perhaps pull it off, but otherwise it's much better for the marksman to have a self-heal and movement speed boost to use exactly when they need it.

Also, the reason more supports are taking Ignite is because Ignite is now filling in the gap Exhaust had previously filled: when the spell reduced resistances, it was useful in softening up targets for the marksman to take down, so even CC-heavy supports would take the spell to enable greater kill potential. Now that Exhaust only applies a disable, it's become more redundant on those champions, who now prefer Ignite as a means of making targets easier to kill. More uptime on Heal is unlikely to be as important as more kill potential in lane, since the game's pretty snowbally, which is why I think supports are likely going to stick to Ignite or Exhaust for a while, just as marksmen will stick to Heal.