Why you want interlaced mode

If a camera shoots in interlaced mode at the same field (frame) rate as progressive, I often go for interlaced.

Why? Because you can then use a bob+weave deinterlace to get double framerate footage, for silky smooth slow motions. Done well, image quality doesn't suffer too much either, especially at high resolutions.

With this camera, you could do a 25FPS movie with quarter speed slowmos at full frame rate. In 1080p HD. Mmmm nice :)