The speed parameter is exported in the bullet script, but it's set by the spell script when it's spawned. Shouldn't it be not exported then? is it right to think that way?
The bullet could be used by things other than the player spell, such as hazards or enemies. In those cases, you may want to control the bullet speed directly on the bullet scene.
Exporting a property is just a means to an end: if you may want to change the value in the inspector at some point, you can export the property. You can remove the export annotation if you don't want to do that.