Shooting 30p is meant so you can grab very high resolution stills from your video. I haven't heard of any other reason to use this setting. Did you mean 24p? Probably not because I believe that difference is the reason why everyone bought the camera in the first place.
Advantages – looks more filmlike than 60i without the motion touchiness or pulldown issues of 24p. It’s progressive, so its vertical resolution is higher. Definitely much less of a video look.
Disadvantages – unacceptable for film transfer. Not quite as smooth in slow motion.
When you require an NTSC formatted output with maximum vertical resolution, no interlace artifacts and easy conversion to still frames. For example videotaping of legal documents or police forensics / evidence recording. The 30p capture mode is also ideal for the production of streaming video or downloadable video or as a component of flash animations, due to its very high efficiency in compressing to a low bit rate, and compatibility with computer graphics software applications.