Busters are there to render your tank ineffective at soaking up more damage. They either put them in crit health, slap a vuln stack on them, or both; depends on the content. The healer buster would have to do something similar to a healer, and basically compromise their ability to heal. A great example of this was the Cleric Stance penalty of old. For those who weren't around, or can't recall; Cleric Stance used to greatly increase a healer's offense while handicapping their healing potency. Similar to engaging a tank stance now, if you turned on Cleric Stance, you could not turn it off for five seconds. So, if you turned it on right as the tank receives a big hit, or the party is taking damage; chances are someone's hitting the floor next. This is why it was basically a really good idea to have it on by default.

While this is interesting to have a boss do this to a healer in an 8-man, there are several issues with this: One is the game already compromises a healer's ability to heal in several ways; they just typically aren't mechanics that affect the healer directly like a tank buster does to a tank. The next is it forces a healer into a situation where all they can do is dodge and DPS, which is kind of already the situation for them.

An issue for healers is that it is typically the players that push their healing capability and not the content. It is quite the conundrum because it is that very same reason why SE has to be careful about how far they go to design content that intentionally stretches a healer's resources.

In short, the enemy is already trying to kill everyone, so a mechanic intended to take away a healer is basically what they deal with throughout the entire encounter. Especially fresh content where an AoE will usually one-shot them. Exploring and finding good mechanics for healers, will likely involve their kits. Those who currently enjoy a job like AST will likely feel like they have plenty to do to keep them busy, and feel it is rewarding. Whether or not you like the card mechanic, the point is giving healers interesting ways to build and use their resources. This is something the devs vastly need to improve on.