I'm going to take this opportunity to post my rune speculation.
My theory is that all runes draw from the same unified source,
and all runes absorb ambient winds. Basically all runes have at least two functions: absorbing ambient magic, and whatever effect they're supposed to produce.
All that magic is sent to a single 'place' (which may also be the Dawi afterlife, but it doesn't really matter). Then all runes use that energy, maybe transformed in some way, to produce their effect.
The only difference between large runes, or rune assemblies, and small 'everyday' runes like simple light runes, is the amount of rune magic they consume. Or rather the difference between how much they take in to how much they need to produce their effect. A small light rune may very well break even, but the huge impressive wonders we heard about in Thorgrim's interlude need way more than they could ever bring in. Not enough ambient magic would even reach those runes.
And this is where the waystones come in. It's an enormous amount of magic that is channeled straight into whatever reservoir runes draw on. No issues with the low energy density around the physical runes; no, or very little, additional drain on the reserves. You can just power everything you want, who cares if it uses up way more than it adds? It's not like so many Holds would ever fall that it would become a liability.
Anyway, this is really is mostly speculation based on bits and pieces I may well have interpreted incorrectly, so be gentle.