How would the tar from Tar Pit be any different from regular tar? Spring Water (allegedly) counts as higher quality when planting trees (made obsolete with upgraded Wells) and is required for Camomile Compress.
I like the idea! Have them do the same as Spring Water but instead for things that require Tar. Have the Tar pits be very rare such of 4 or 5 on the whole map (since I see the only use being for Knarrs). Also have them scale through out the world so it'll progressively get better.