Are spiders ever worth it to trade for coal? They cost scaffolds, which seems pretty expensive. (I'm mid-game I guess; science 125K.)
That's really up to you. If you're more wood focused and ran out of stuff to build with wood, then yes, it is worth it. If you have no problems getting coal but frequently run out of wood, then no, it's not worth it.
It really depends on your resource generation.