The problem with the lead line when used from an aircraft is the temperature differential, it was found that trying to account for the 2C change per 1000 feet made the lead expand and thus the readings difficult...
On the other hand, the lead line would make navigation easier. Fishermen in the old days used to feel and taste the silt that came up with the lead weight, and used that to determine wherever they were.
I can imagine that being useful today. Throw the lead weight out and smell, taste, feel when it is brought back in. Wet - you must be over water. Covered in glass shards - you must be above an area with lots of greenhouses. Smelling of weed - you must have been above Amsterdam. Cleaner than when you threw it down - you must be over Switzerland. And so forth.