That's an interesting perspective Captain. Many of the haggs are neither infertile or barren especially in comparison to the nearby moors and bogs. The haggs are gorsey and are used to graze sheep upon. I think the Hagg could possibly refer to the type of vegetation that is found on the land as opposed to it's fertility.
Close to the area Barry mentioned is a village called Boosbeck, I received an e-mail from someone who was excited about the name and it's connection with the deity Bu, unfortunately the name translates as 'stream by the cowshed'
When I'm home I shall have a look and see if any of the Haggs have been under cultivation in the past
Cheers
Fitz