Wow. I had no idea this feat existed, and have accidentally written a campaign arc that completely plays into it. The next arc of my game has the party hiking about 100 miles through the underdark to get to a ritual site and stop a ritual in just under four days, avoiding travelling above ground because they're wanted fugitives in the area.
At 5" moves, they have enough time to take 5 extended rests (10 hours hiking, 8 hours resting). The encounters the party will have along the route are mostly fixed in location (with one moving encounter, a trog hunting party that finds their first campsite in a given range, and an optional black dragon in an underground swamp lair, near the end -- they'll know about it in advance when they make a Religion DC against a Bloodrot, and if they're running late, they can skip it).
The party will be levelling up to 4 one game session into the arc, giving them a feat choice, and there's an elf and a half-elf in the party. If one of them takes Light Step, that gives them a _lot_ more flexibility in when and where they choose to take their extended rests. That could mean the difference between having time to take an extended rest before fighting that dragon, and it could mean that they completely skip the Trogs, hiking straight through their hunting range without resting.
To avoid completely hijacking this thread, I agree, I would allow this feat to apply on horseback. RAW, I think the horses count as your allies. Of course, I've recently allowed a Demagogue BBEG to use his Clever Escape ability while on horseback, so perhaps my judgement is suspect.
