For major label acts that get plenty of exposure and promotion, they are all but useless.
However, I think for a new indie act they can be important.
The band I manage will be filming two videos for our new album. The first will be a performance video, mixed with scenes of our cover model acting all sexy (ur model is a well known porn star). This will go up on Youtube and I'm sure will reach people that we never could otherwise...
The second video will be a 10-15 minute horror movie....with the final 5 minutes happening while the song is playing. Our director is a very successful special effects artist who has done effects for several blockbuster films. Again, this will reach tons of new people, even though the band wont actually appear in the video at all.
So, for someone like us...a new band without label backing...it can be very helpful. I think that if you have a multi-million dollar promotion and radio push behind you already though, im not sure how a video is really helping.