Interesting how in the Panorama programme they showed Tim Noakes laughing at length at the idea of zero calorie electrolyte replacement drinks, whereas in The Lore of Running (by Noakes) there is a long detailed section on the importance of replacing sodium lost through sweating (4th edition, pp 213-217).
Summary: sodium determines the level of water in extra-cellular fluid; if you lose a significant amount of sodium through sweating and then just drink plain water, excess water is excreted to maintain proper osmolality - ergo, you still end up dehydrated. Therefore electrolyte supplements (eg Nuun) have a role to play.
I think the programme both misrepresented/oversimplified the scientific view and - perhaps more importantly - didn't really give much weight to people's actual experiences beyond setting up a few vox pops coming out of a gym to look like gullible stooges. They did this so they could present a sensationalist picture of credulous consumer-drones being duped by big corporations (again!). Muddled and without much room for nuance.