Well, it’s a kind of status thing for most people. If someone likes what you have to say, they’ll add you to their Fluther. You get two lurve and are listed in the other person’s Fluther when you get added. The more Fluthers you’re in, the more people like you/what you have to say, thus the more the better. Plus, it’s just nice to feel lurved, isn’t it? =)
Another thing is that when you add someone to your Fluther, you can see all of their recent activities.
As for adding someone to your Fluther just because they added you, I don’t. I only add people when I’m interested in what they have to say. If someone adds me to their Fluther, that’s great, but if I don’t like what they have to say or have absolutely no idea who they are, I don’t see any reason to add them as well.