Druid, why Magi?
There are a few reasons why magi’s makes sense:
You’re quite right about the overheal, often you could run magi’s armour, and use zerker trinkets, and still have enough healing.
You might run sword & warhorn/axe along with staff, so you can switch to a more “dps” weapon – augmented by the berserker trinkets you may have equipped.
Typically, if your healers die, your raid is /gg. So a bit of vitality is usually good for ensuring that scenario doesn’t happen. Though too much is a liability since there are so many attacks that do a flat percentage of your hp as damage.
Druid is very much about min-maxing to fit your group.
There does come a point where you’d be better off running a full zerker druid with staff offset for emergencies, or even a full viper druid. But that scenario is a very high calibre group.
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To add a little bit, the main reason why magi is the recommended is because it has the highest healing power with the lowest toughness.
There are so many variants to druid, that it really comes down to what you want/have/can get as long as you are lower in toughness than the tank.
The main reason something like Zealot isn’t used as often is because there isn’t much need for a “middle ground” in the current raids. Either you dont need much healing and can use zerk/viper, or you’re better off going full heals. The DPS gained by Zealot usually isn’t needed, but this can also depend on group.
There are builds that use cleric, minstrel, viper, zerk, magi, etc.