Why is the healer class basically a one spell dps class?

[Dzian]

'Utility' in this game historically translated into one of three things:
1) Resource generation
2) Reduce incoming damage
3) Increase outgoing damage

Players always prefer things that directly boost their dps. AST and cards are the obvious example. That doesn't mean that these are the only forms of utility that exist.
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Not necessarily players tend to prefer depth and complexity over simple and shallow.

If utility is interesting or changes encounters in some way thats fun its usually something players like.

1.2 paladin for example had a cooldown called aegis boon. Super fun and interesting to use. Blocked 100% of the damage from the next attack you took and converted that damage into HP.. a successful block of the right attack could literally take you from single digit hp to well over 50%.

If you compare that to Shelltron guaranteed block but only like 20% damage reduction... well that's just nowhere near as fun or interesting to use.

Old school cover was much cooler as well as it carried your buffs onto the covered target. So if you popped tempered will and covered someone. They couldn't be knocked back either. If you popped hallowed and covered someone they were invincible. Made these things so much more fun and interesting to use. Where now there all just shallow and boring..

[Lyth]

...

The more clever that you become with ability effects, the more opportunities there are to break fights. Rather than try to predict these or patch them out, they make your ability set just a little more narrow and a little more tightly constrained the next time. Hallowed and Cover as you mentioned being used to reset stacks are good examples of these. Another interesting example is Holmgang being used to interrupt otherwise uninterruptable abilities. That's one major reason we don't have draw in/knock back abilities in PvE anymore.

Utility doesn't need to be clever to not fall into one of those three categories. Movement is a major one. An in-combat movement speed buff could be used to help you respond more quickly to a mechanic, or it could be used to improve your uptime. Similar thinking could be applied to your casters if you gave them a warp point between two parts of the arena.

What else could you do? Reduce recast on next cooldown. Prevent/cleanse a vulnerability stack. Increase resource generation. Haste buff. Reflect damage.

The more complex you make it, the bigger a dev team you need to balance it. They're just cashing in for cheap.