You are starting the wrong way in my mind. A explorer (upgraded scout) is not needed in civ as long as there's nothing to scout anymore. The Caravel is the scout unit of the Renaissance, for everything land-based, why wouldn't I chose a Horsemen/Knight?* If you give it too many bonuses, why wouldn't I make a army of explorers to conquer my neighbour/defend against him?
*movement through rough terrain is not really a problem since mobile units can more often go around those. It's one factor, sure, but it's imho not enough for a new unit.
But the main point is that as civ5 is currently set up, there's no need to go explore the other continent, it doesn't have new luxuries nor strategic ressources I can't get any other way. Adding a new city doesn't normally give me something new I couldn't get by just adding the equivalent numbers of cities in my home town. No, they are even better there since multipliers add up. This is a result of the Global Happiness system.
I have some hope that the new trade system can alleviate this problem, but it at least needs a rework of the map creator (it's not that hard to focus some ressources in some areas or make the bonus ressources worth collecting).
So before this is fixed, I don't see the need for an explorer. And even after it, there's the problem of differentiating it from mobile units? (upkeep costs?) Each new unit should have passed the question whether it really offers something new to the gameplay. It's the same with Drones mentioned in another thread, don't the normal air units achieve just the same gameplay result? Or how would the proposed assault line be different from the Swordman and Spearman?
Where I see the need for new units is where the units jump units line (pikes, lancers, etc. ...). That includes practically all naval units. The naval unit system could use some rework, that's true.