Couple of thoughts...
I always play with ancient ruins off and consequently never found it practical to build a scout at all- early warriors can peek outside the perimeter, finding a suitable second city site, a neighbor or two, and nearby CSs. With HBR, usually build 1, maybe 2 horseman to explore the rest of the map. Multi-landmass maps are finished with caravels.
But scouts historically have provided strategic advantages to the garrison they're scouting for, often making the difference between victory and defeat. From finding defensive positions to advising of the composition of enemy forces to the most obvious- informing the garrison there's a big ol' stack of nasty coming right for us (example set by the Civ 5 GreatScout - Paul Revere)
Here's an idea that kills 2 birds with one stone - all units suffer a defensive penalty, perhaps 20-25%, if attacked by a unit that is unknown at the end of their turn (i.e. in the fog of war), or perhaps if attacked by a unit that is 3 (4?) tiles away, and this penalty is negated if within 2 (or 3, 4?) tiles of a scout. First off, gives the scout a purpose in combat. Second, the trend on the forum for a while now has been either archery-based assaults, taking advantage of the hammer efficiency of pike units, or the ease of naval warfare. Mounted units seem to suffer in strategic application, so much so that people often sell horses for 1GPT. This would make mounted units, although still requiring some careful (tedious?) planning, more viable.
On a related note, I'm playing the Shoshone for the first time and found that I have a pathfinder instead of a warrior. Since I play with huts off, I'm obviously losing out in the greatest attribute of the civilization. I'm pretty sure that I'm not the only one here that plays with huts off, and it would be great if there was some application of the unique unit that applied to hut-free games, and seeing this thread gave me an idea that i think would be really cool - I know that some of the huts offer advanced weaponry, turning a warrior into a spearman and most effectively turning a scout into a scout/archer. I think it would be cool if the Shoshone (and that civ only) could upgrade their scouts to scout archers. Before saying that it makes a powerful civ OP and that you should play with huts on the appreciate the civ, keep in mind that pathfinders cost more hammers than archers, 1/8 more in fact, and you'd also have to invest gold to make it happen.
Finally in regards to revisiting the explorer idea, Kamikaze pointed out that the designers had it in Civ4 and intentionally left it out of civ5, and there must be a reason for this. The reason is that we posted over and over how the explorer was the most insignificant and ineffective unit of Civ4, WE decided that explorers don't belong in civ5.