Originally Posted by Bman

all of the comes from film cameras...before you needed the shutter to be quick enough to get a sharp image. you were dealing with film stocks that needed tremendous amounts of light and so this was way more of an issue of getting everything right.
today just look in your camera. if doesnt look good then change it.

you would use 60i to capture fast motion objects or if you wanted to slow down the motion in post after(as it has 2 x more frames so you get better transitions between the frames)
30p is good for regular stuff and you essentially need less light because your shutter is set to 60. plus the picture is progressive and not interlaced.