It boils down to priority healing. If you can keep people alive without using GCD heals then it’s better to do so because that means you can spend those GCDs doing DPS.
When you need to use a GCD to heal you’ll try to get as much bang for your buck as possible, usually this means regen for single target.
This means that you’ll always use your ogcd cooldowns first, benediction, assize, tetra, benison, asylum, PI if you had stacks. (Almost all of which consumed lilies of you had any)
You always use holy and e4e to mitigate damage in dungeons.
If you know that won’t be enough you’ll need a gcd or have to heal extra you’ll largesse regen.
When all of that fails then you have to ask yourself if you’ll need to heal the entire party with medica I/II, cure III and can hit your single target with that in the process.
Once you’re out of everything, or what you have off cooldown won’t be enough you’ll use cure II. Unless you’re having mana management issues you’ll never use cure I.
So basically your level 70 whm skill required your last resort heal to even have a percentage of chance to proc a lily (later changed to 100%). And what you could do with it was shorten cooldowns in an uncontrolled way. It was hard to even get more than one lily and even if you did it served little purpose. Even moreso in raiding context where all your heals are planned and timed.
Many a healer removed it to free up space in their interface, i always kept it but wouldn’t be capable of telling you when or if I had any after they moved benison off the lily system.
The above hasn’t changed in 5.0. Some skills have changed or been added/removed but it’s the same principle. Except the lily system is now good and takes priority over other heals for the most part thanks to misery. So yeah now you should be burning your lilies and cooldowns.
It’s not just a whm thing, it’s for all healers. Sch avoid shielding with succor or Adlo unless absolutely necessary for the same reasons. Astros just play whm instead.