Those fireball and greater fireball skills are great for leveling, but obviously only if you dont have a good class skill you want to spam instead. Again it really depends on the build.

These are the minimum level 75 seals. "Leveling" is mostly done at this point.

It's not that they don't do good damage, it's that;

There are seals with buffs, and that's better. They just are.

Even the level 20 Enchanted Flint is better, because it's 'always on' helping.

50% fire/light damage, and 50% burn/electrocute damage vs;

25% fire/burn damage on the flint, but then the buff gives 75% to those, for a total 100%, plus 120ish flat damage, etc.

There is never a case where the seal of Destruction wins.

I tried them all, crafted them all, and Enchanted Flint for a fire (or fire/light) build, simply gives more damage, is always on, and doesn't take up another skill bar spot (which I don't have).

I think you are over-valuing the damage buffs

Seal of Might, is primarily a defensive buff. But it's got 4% phys res, etc. which again, is applied via a buff, always on etc. and outperforms.

It's not about damage/defense, it's very simple;

Buffs > Active skills.

The skills deal good damage, but I've already got enough skills, and yeah defensive stuff, or just passive/buff stuff is better than yet another skill dealing damage.