You need to look at this in the scope of other stats as well. Toughness also greatly improves the effectiveness of healing. If you want to maximize your healing you need to also maximize your toughness.
For example. If you 0% damage reduction a 1000 point heal is worth 1000 points of damage taken. If you have 50% damage reduction that same heal is now worth 2000 points of damage taken. This is of course an extreme example of how + heal and + tough can be combined to make yourself beefy.
You also need to look at it in the scope of game mechanics. * In general* there is not mana, just cooldowns. This means if you can survive your opponents first 10 or so spells their spammable “one” skill does very little damage comparably. This makes your last few HP more valuable than your first few.