How do you justify using Stormlight-infused gems as money more valuable than dull gems? I can understand why gems - even dull ones - would qualify for currency: for the same reasons we value gold, diamonds, etc. But Stormlight itself is only valuable to beings that can use it. For most other people it's just a source of light if you don't want to use fire.
Why are Stormlight-infused gems used as a currency?
"It's just a source of light if you don't want to use fire"
I think you might be underestimating just how valuable light is. In our modern world we can happily stay up until 3am doing whatever-the-heck-we-want, but that's only possible because we have light whenever we want it. In earlier times a lot of people had to finish their days and get to sleep a lot earlier than we do. If you can't see much, you can't do much
If you exchange a lot of dull spheres for a smaller number of infused spheres then you're essentially paying money for light, but paying money for light makes sense. Yes, there are alternatives like firewood, candles, or lamps. But those things aren't free either; they cost good money, just like the sphere-light (or they cost you time and effort, if you gather or make them yourself), if people are willing to pay for those things, then I expect they'd also be willing to pay for sphere light. It's clearer than firelight too: more consistent and less flickery
(It's also been mentioned that Roshar has a lot more oxygen than most planets, which makes fire more dangerous, and therefore less appealing)
But the other factor is that a pristine gem leaks stormlight a lot slower than a flawed gem. And just like on our world, a pristine gem is worth more than a flawed one
So a drained sphere isn't necessarily less valuable than that same sphere was a week earlier when it was full, but it might be less valuable than some other sphere that you charged up on the same day and is still going strong. And if you have a sphere that can't hold onto its light for more than a few days? well it's either broken or it doesn't have as much gem in it as it's supposed to. That's why Kaladin wasn't happy about getting payed in spheres that went dull so fast. He thought Gaz was paying him with counterfeit spheres
(It also does have at least one use that doesn't require the person to have special investiture powers: stormlight can power and repair ShardPlate)
Found an answer to my own question on the Stormlight Archive wiki:
Spheres which run out of Stormlight are called dun spheres. Infused spheres are not worth any more than dun spheres, though sometimes dun spheres are suspect because it is more difficult to see the gemstone inside them[3], and sometimes a moneylender would have to be brought in to judge the authenticity of a gemstone.[2]
They're not worth more, they're just used because it's easier to prove their authenticity when they're infused with Stormlight.
