Here's my new metaphor for IEI-SLE duality:
I personally prefer to use the metaphor of a child holding a balloon while running. IEIs are the balloon, and we provide enjoyment and entertainment for the SLE. In return, we get the feel the sensation of the wind rushing over us without the unpleasantness of trying to move ourselves--and it is very difficult for a balloon to move itself. IEIs are always floating anyway (Ni), and SLEs are always running anyway (Se), so nobody has to fight. It's natural. Plus, from our vantage point way up high, the IEI balloon can tell the SLE what obstacles are coming up without the SLE having to stop running--and I suppose it's difficult to stop running when you've built up that much momentum. And to us, we're just describing what we see, and we're excited because we're seeing so many new things. If we float up too high, and we're in danger of flying away, the SLE can bring us back down. If the SLE is about to run into a rock, we can warn her.
Also, I realize that I make IEIs look bad in this description, but I think it's because in duality, each person is convinced that the other got the short end of the stick. When I think of more duality metaphors, I'll post them.