What the Gods want...

Generally I don't have any complaints about clerics themselves. I do, however, think that their role as a part of fantasy worlds' society is greatly undervalued. When you have somebody who can, as a beginner in his line of work, completely heal at least two to four commoners of injury every day then the meaning of risk and injury takes on new meanings. In a magical world clerics are essentially foolproof and risk-free doctors, or combat medics depending on the deity they worship, and a mundane understanding of medicine will never flourish or progress if society has a subset of people who can merely pray for injuries to be healed. This isn't even taking into account that at higher levels healing powers become astronomical in how much can be doled out, a high-level cleric could keep a single village alive all on their own, and two high-level clerics in the same village will have enough healing to spare that regardless of the village's worth it will begin to prosper and grow.

I'm not just picking on this discrepancy in D&D, many other game systems allow for healing on a massive scale and never show any effects upon society as a whole for these walking hospital clinics being the go-to people for injury, disease and poisoning. In most campaign worlds, clerics are servants of the deities so it might be rightly assumed that the gods themselves have reasons for not turning their clerics loose as all-purpose healers for their community. But this assumption is just that, an assumption. Nothing is ever explicitly stated about the gods object to this kind of behavior, and from the gods' point of view this might actually be the best thing for their priests to be doing since having agents who can touch people and instantly fix a broken bone or banish leprosy would make that deity quite popular.

I've heard an argument once that a deity wouldn't allow healing to be doled out to just anybody and that the cleric would probably only be able to perform these kinds of duties for people who specifically followed his own deity. But this argument fails because the deities of pantheons are usually described as working together to fight against a single evil god, or small group of evil gods. In a pantheon of deities that work together it simply doesn't make sense that they would withhold magic from followers of their compatriots. "If you Heal my follower today, I'll Wind Walk yours next week." In a more realistically selfish pantheon, like the ancient Greek gods, it could be justified since the old pagan gods were quite jealous, vengeful and arrogant.

Then you have to wonder about each individual deity's goals. Wouldn't they want their clerics healing as many people as possible because then they're potentially picking up new followers, and new clerics? It's a feedback loop. I could see a deity manifesting at a farmer's house when he gets a toothache and 'cure' him just so he grows more food and shares it freely with others all in the name of said deity. Instant worshippers! If others start seeing that the followers of this deity never go hungry and never suffer the aches and pains of living then it would be on a fast track to being the the most powerful and influential god around. Screw the adventuring heroes! Let them get eaten by dragons! If a deity needs to smite something to protect their followers it'll just show up and do it on their own!

All of this assumes that deities' want followers. If a deity has no need to be worshipped then a lot of this logic falls flat. Check out The Primal Order if you can find it, it's rules for role-playing deities.