I'm glad you asked this because I've been wondering the same thing. I have two girls - 3 and 4 and I still have a monitor in each of their rooms. We have a 2-story house, so I like having the monitors on downstairs after their bedtime because I don't think I always hear them if they call for us, especially if we're watching TV and it's a little loud.
We also have the second set of monitors in our bedroom, which we use when we go to sleep, but it's probably unneccesary because their rooms are so close to ours. However, my husband, when he's home, likes to nap when they do and I take advantage to run errands. He sleeps much heavier than I do, so I always make sure the monitor is on so he'll hear if our little one wakes up calling for us.
I've been considering getting rid of one monitor and instead of having in their rooms, just use one and place it on a small table that's in the hall between their room and ours. Wouldn't really give us any benefit, so I'm not sure what to do. I am tired of using the monitors, but can't let go yet either!
Sorry, I know this probably didn't help you much - I guess your question inspired me to "get it out!"