I have and use a scale and definitely find it helpful, but I wouldn't say that it's necessary. If I didn't have it, I think I'd bring pretty much all of the same stuff that I do now. Sure, there's a few decision I might make differently and my pack might be slightly heavier as a result, but probably not by much.
Regarding manufacturers weights, as long as they are in the ballpark and I can compare relative weights then that's enough for me. My sleeping bag is about an ounce over the listed weight, but knowing that doesn't change my decision to take it with me or make me want to return it and buy a different one, it just makes my spreadsheet more accurate.
I guess my point is just that they are helpful, but not essential.