Quechas are pretty useful in multiplayer. Just build a couple of them and attack. If you are lucky, you can take a city or two, but the trick to this unit is different: Just park them outside the enemy capital on a hill or in a forest (forest on a hill is perfect) and leave them. If done quick enough, the enemy will have real problems getting rid of them - Archers don't stand a chance against them, and against enemy warriors the defence bonus from the terrain should be enough. You start with Combat I anyway, so you should take Shock for the next promotion. Now someone will say "one axemen and they are going down", but if you are quick about it, you can hamper the enemy expansion quite bad, and that includes the copper/iron they need to mine before there are any axemen...
Especially on tiny maps, this tactic works very good. Just block 'em while you are building axemen of your own. To harass the enemy, this unit is very good. Of course, it can also be pulled off with normal warriors, but the bonus against archers makes it even better. Apart from that, the Incas are Agg/Fin - that's a really good combination in MP.
Btw, the same goes for the Keshik - sure, it is utterly beaten by spearmen, but there is one thing: Because of the movement bonus, you can attack the enemy pretty much anywhere without him expecting. Don't try to attack his heartland - go for his newer cities. He won't have that many roads there, so he can't use his spears fast enough. Just try to outmaneuver him, steal/destroy a worker or two, or pillage some resources. And remember, your enemy is human, so it'll mentally distract him, maybe causing him to make a mistake or two. I've seen this often enough, especially when your main force attacks somewhere else. Can also be pretty good in team games, just harass the guy in the rear who is sending reinforcements to the other guy. Pick off a lonely axemen here and there, destroy a farm here and there, just keep him under constant pressure - and the spears he'll send after your Horsemen won't be at the front where there might be much more need for them...
Don't just look at statistics, look at how the game is played. And as some people already pointed out, Civilization plays completely different in MP as in SP. I had to relearn quite a lot of things in my first online matches, but now I really enjoy the different kind of challenge.
Sorry, that turned into quite a rant. So, to make my point for those who just skipped everything above:
Quechas and Keshiks = Not so bad at all 
