Shoots a series of 1080P HQ Jpeg pictures at 25 FPS, (taken directly from the Live view buffer). Images sequences are easy to convert to .mov, easy to playback on most computers & bypass the 4 GB limits, also less complicated than trying to create a MJPEG movie mode.
Use silent pic in burst mode ;)
Well, you'll miss the on-board audio or external audio (where available). Not a big loss, but then syncing becomes a PITA (i.e. you can't use plugins like PluralEyes) unless you use a clapper.
The only benefit I see from an MJPEG mode or other image sequence is higher than 1080P recording.
I'll leave that to A1ex, bit as far as I know that's not possible since you are limited either by the burst rate of your camera or the FPS/resolution of the buffer used in silent pic mode.