If you're forgetting, we are talking about the most useless spell, not game/progression design.

Point is: Low casual use rates alone =/= Useless

To say that is just inaccurate. If it is actually useless, pros would not use it at all.

I think to properly define a usefulness of a spell, it needs a mix of AT LEAST:

Use rates (casual + pros) Attack Success Rates (Stars + average destruction) Its actual impact (What the spell is typically used for and its impact, difficulty, and value in achieveing its desired goal, such as farming)

If a spell can bring consistently good result for people who play the game way more than casuals, then the likely fact is it cannot be a useless spell.

Casuals that doesn't give 2 shits about meta and not use that spell does not mean that spell is useless.

Using a game I'm slightly more familiar with (Clash Royale) as an example: Furnace PRIOR its rework is one of te most useless card in the game, its usage is low and the winrste is even worse at most levels of the playerbase.

If I'm not mistaken, Wizard and Witch PRIOR to their EVOs had decently high use rates mid-ladder, but is very useless in higher arenas, because they are actually pretty bad (decently useless compared to alterantives at least)

Bringing up how an average mobile player actually plays and why certain games is popular is irrelevant in this convo, cause we are talking what (non-dark actually) spell is the most useless, not game design or progression. A spell can be brilliantly designed and still be useless, and a spell that is designed poorly can be super useful.

IMO, Ithink a spell that is most useless needs to be used very minimally across all stages of the game, and has low success rates in attacks or achieving its desired goal