Healers aren't bad design - it's just very easy to make awful healer gameplay.
That is to say, consider the "holy trinity" in contemporary RPGs: Tanks, Damage-Dealers and Healers.
Tanks can be made interesting by giving decisive options for dealing with incoming damage, or otherwise providing your team with breathing room.
Damage-Dealers can be made interesting by giving them a lot of variables to control to put out the most damage - and making enemies give them things they have to deal with to make engagements go smoothly (like abilities to interrupt, or buffs to remove).
Healers are at their worst when they're just "healing" though. Healing is important, because people make mistakes, and the nature of RPGs means that some damage is guaranteed (such as to the tank), and that's fine. However, in the same sense that you could make tanks able to have decisive, instantly-powerful damage mitigation tools to manage, you can do the same with healers: A very-temporary-but-potent barrier to absorb damage on someone is a common good example.
There's a running theme throughout all of these though: Reward players for optimization, reward them for being proactive, and reward them for leveraging their situational skills well... And make sure you actually provide opportunities for these to happen.
If you'd like to see a couple games with legitimately good design for healers, look at Final Fantasy XIV and WildStar.
In the case of FFXIV, you have Scholars, who have the lowest healing potential of any healer - but have the greatest means of mitigating damage out of any of them: Their primary skill is dropping a big damage-absorption barrier on someone, they can drop smaller barriers on the whole group, they can buff an ally to debuff the damage of enemies that hit them, they can directly debuff two thirds of an enemy's stats in one go, they can spread their barriers around (as well as their reflective debuff), they can place fields down that reduce damage taken in them or slow enemies in them, and they have a pet fairy that's dropping small heals on people constantly as need be, and has its own myriad other support functions it can do (depending on which type you have summoned).
All of this, used well, minimizes damage taken by their team, and by extension, minimizes the time they have to spend recovering damage incurred by the team... Which makes them free to deal as much damage as they can - which is another key point about FFXIV's design: Tanks and healers are capable of putting out significant damage, given the opportunity - so encounters go by quicker by affording the healer breathing room, because it allows them to safely pitch in damage.
Astrologians are also pretty fantastic on the utility side, capable of doing some very similar things to varying degrees (some of which is subject to RNG, due to the card-drawing mechanic they have for buffs, which they get more and more means of leveraging those in their favor as they level up). White Mages have the least in the way of this sort of utility, while having the most raw healing potential (and raw damage potential, they just don't have much in the way of making time to use it).
With regards to WildStar: The interesting part about combat design in this game, is that basically every role has the same general combat loop. Everyone is very active, very frantic, dodging stuff left and right, manually aiming skills (no auto-attack here, or much in the way of "traditional" MMO tab-target abilities), stunning enemies (which, successfully interrupting a cast makes everyone do magnified damage while the enemy is stunned - a wonderfully-fun and rewarding design choice), and managing their resources in the fast-paced utter chaos that is combat. It's hectic, dynamic and very action-y for everyone, even healers... And it makes up for the fact that gameplay is mostly just making numbers go up on allies (because it's pretty much identical to the gameplay of making numbers go down on enemies - which is fun for all of the above reasons in the first place).
tl;dr: Healers are great if you make it so that being "the healer" is about more than just purely making numbers go up - just like how being a tank is great if it's more than just purely "being the one whose numbers go down" and how being a damage-dealer is great if it's more than just purely "making the enemy numbers go down.".
