I admit, I've never played a multiplayer game, since I hate playing online in games like this until I know I can handle myself against better players, but I do know that outposts can be useful.
If my villagers are far from my base, they're easy pickin' for any enemie in the area. By building an outpost near by, I can buy my army some time to move over there to defend them. If it's just a couple of enemies, I can probably kill them without too much trouble. Once I no longer need that area, the outpost stays and will tell me when any enemies are coming.
Sometimes I'll place them next to trading posts, just as a warning mechanism.
Another use I like is building a web of walls to protect my bases and using well-placed outposts behind the walls to piss off any attackers.
That's just me, though. I use the French and I use some of their villager-improvement cards so they aren't that easy to kill, and they can easily kill off any rushing army when they're all huddled together, so outposts aren't terribly important to me.
[This message has been edited by Aria (edited 02-16-2008 @ 08:40 PM).]