Well there you have it, varied opinions, and I agree with all of them. I already knew from past comments that Cheryl would pass on a piece in this condition. Her iron is all very valuable and collectable and her skill at restoring them is second to none and I would never advocate the use of any power tools on pieces like that. It just isn't necessary and would also devalue the piece. This piece is not one of those. She would walk on by, because she knows you can't fix pits with lye, electro, vinegar, derusto and elbow grease..the tools of her trade, and I also know any power tool will not be one of them. So she only buys pieces that can be restored with those methods, or it's an otherwise exceptional peice like she said. But you've already put a lot of time and effort into this piece, a project so to speak, and good on you for trying. So I can't agree that you would be "further destroying" a heavily pitted piece that otherwise would just die in a scrap heap somewhere if nobody at least tried to put it back into service.
I also agree with Chris, and said the same thing earlier, that certain foods could be cooked in that skillet as it is.
But I mostly agree with James and the flop sander on this particular piece. If this was my project, I would finish the project, if for no other reason but to see if I could do it. When I cook a steak in cast iron, I like to deglaze with some red wine, butter and a little garlic for a simple sauce to pour over the steak (and potatoes, yum). I scrape up the burned bits with a flat spatula to mix it all together. That would not be possible with that pan as it is. I suppose you could do it with a stiff bamboo brush like they use for cleaning woks or something similar.
I would be careful about doing the outsides too though. If you try it, you're going to be removing quite a bit of material to get it smooth inside. It may, or may not be, thick enough then to do both sides...you wouldn't want it to be too thin. I probably would do a little on the handle though. ;)