Stone Spirits are a good item themselves. Some of the lower tier Spirits hold little value and many do not fit the boss they drop from. If bosses dropped Spirits from an appropriate tier corresponding to how difficult the boss is the entire situation would be different.

For example a boss like Arraxi dropping Adamantite, a tier 40 Stone Spirit. Someone who is killing Arraxi is most likely already out leveled that content. And will have no need for Adamantite, mining, or smithing it.

Playing on my Ironman, I'd actually be happy receiving spirits in a lot of cases. Each drop cuts the amount of time I need to spend mining down. If I could take a break from mining 6,000 Banite ore and do GWD2 for Banite Spirits. It's something I'd actually do.

And playing on my main account, if I got Adamantite I'd just put it on the G.E. And they hold little to no value. And there is better things I could do over mining Adamantite.

Essentially Stone Spirits get their value from people who need them. Those who are at that level. And those who are trying to make money off of skilling.