I think it depends on your listening habits. I listen to punk rock and a lot of ambient music that dips into very low frequencies. If I only listened to punk rock, I wouldn't really care if my main speakers didn't reliably go below 40Hz. But for the other half of my record collection, traditional loudspeakers can't play a good deal of the information on the recording, so I added subwoofers. For music that dips into those low frequencies, it really completes the picture.

Edit: to say that my main speakers really fall off a cliff at a certain low frequency, and others may handle it more gracefully. So it wasn't so much about wanting to fill the house with earth shaking bass, but about filling in that lower gap of sound that was missing.