I think one of the bigger problems with the recon unit line after scouts is that they're so expensive to build relative to what they can actually do. There's just no reason to build skirmishers or rangers over any other unit.
For example, let's compare the crossbowman with the skirmisher, which shouldn't be too difficult given they are unlocked at the same tech.
The crossbowman's base stats are as follows:
And here are the skirmisher's base stats:
The skirmisher is indeed cheaper, requiring 17% less production to build, 17% less gold to purchase, and 1 less gold per turn to maintain. It also has +1 movement relative to the crossbowman.
But, the skirmisher also has -10 (-33%) melee strength, -10 (-25%) ranged strength, and only 1 range. This reduction in power is far greater than the reduction in cost. The extra movement is not valuable even for pillaging given how weak it is at defending if it were to enter enemy territory, and it is outclassed by cavalry units which have even more Movement to be able to move through enemy territory and still pillage and are also much stronger. Indeed, as the OP said, light cav can outclass these recon units for whatever you'd be using them for, combat-wise. You'd be forgiven, then, for thinking that the skirmisher must have a role outside of combat that it can fulfill. But, at this point in the game, you've probably already done a lot of exploring, and if any land remains unexplored, you should have no problem using a regular ol' scout to explore it, which is far cheaper both in production cost and maintenance... and exploration is the only other thing that the skirmisher could feasibly be used for. You don't even need to use a recon unit for this purpose. Incidentally, I also sometimes just use an old heavy chariot (yes, cavalry units outclassing recon once again) to explore if it's a location rife with barbarians.
As others in this thread have already shown, there are a few things that can be done to make them more useful, and it doesn't have to be limited to a combat buff, which could make them possibly viable in warfare but wouldn't really serve to differentiate them from units focused on combat rather than exploration. I think I would like to be able to use them for warfare as a raiding & pillaging unit, but I would never choose a skirmisher for that role given how expensive they are - I'd get far more value out of my production by just building a cavalry unit. Again, though, it would be nice if they were more influential in exploration as well, whether that means making later-game exploration more valuable or simply giving them extra abilities.
