Why should I use combination runes in combat?

[Ranwyr]

As I see it, combination runes just make a spell more expensive to cast? For example take Earth Strike with 1 air, 1 body and one earth rune. A staff reduces that to 1 air and 1 body rune. If I change to combination runes it costs 1 dust and 1 body. But the dust has to be built first, so it actually costs 1 essence, 2 air, 2 earth and 1 body. The staff reduces nothing.
Am I missing something?

[Pandorian]

One handed wands for when you want to wield an off-hand.
Also with higher mastery and bonus runes received your essence gets stretched a lot further to the point where it can be cheaper essence wise to cast a spell with the combination runes over using their normal counterpart.

I usually don't use them for casting myself, but you do need them for Summoning tablets.

EDIT: Also higher level spells and alt-magic spells. It's harder to see their use when looking at the low level spells but when you need 40 Air, Earth, and Fire at the same time it becomes more apparent.