My cam has one of those "photo" buttons. When in record standby mode, pressing that button will take a snapshot of whatever is out in front of the lens and print it as a still over about three or four seconds of tape footage. I'm not certain where the advantage to that setup is for me. With Vegas (and I know also with Pinnacle's Studio DV/Studio7, and probably products from many other software makers) I can grab any frame that strikes my fancy and turn it into a still pic.
I find I get better results if I just squeeze off three or four (or more) seconds of live video and decide which frame is best after I get back to my PC . . . no closed eyes, and I can review and select that special expression, the perfect smile, etc.
Of course, the above is only my opinion, and, I certainly know far less than "all of it" so I'd be interested in someone else's perspective.
some cameras store the photos on memory sticks so you can grab the photos without going through a DV tape.
other's like mine just hold the frame for a few seconds. there is a way to search through the tape for photos, which is one advantage to using it. I just use it for fun if I want to talk over a frozen image for a few seconds.
there may be some technical advantages concerning the way the CCD is used when taking the photo, but that's just an idea.
I agree with Caruso, it's much better to just shoot video, especially if you are shooting progressive, and then you have 30 photos per second to choose from!