This should have been a comment to the post by @thomasrutter, but I can't comment yet (lacking reputation).
Your bullet point about MJPEG versus AVC is not entirely correct. It is true that MPEG-4 Part 10 (AVC /H.264) usually doesn't record every frame as an I-frame (key / full frame). There is nothing in the specification that prohibits this though, and some cameras do record only I-frames.
What you are wrong about is the benefits of MJPEG over H.264, and your argument about why. I'll start with a short explanation about how the frames are reconstructed from a H.264 (or other video codec formats). The video will be coded in three different types of frames, intra-coded frames (I-frames), predictive-coded frames (P-frames), and bidirectionally-predictive-coded frames (B-frames). An I-frame is a compress version of a full "raw" frame. It can be reconstructed just as a JPEG (or a single frame in a MJPEG stream). A P-frame on the other hand reference preceding I- and P-frames, and will need these to be reconstructed. A B-frame can also reference following I- and P-frames. It's a lot more information about this on Wikipedia.
The difference between H.264 and MJPEG is how the compression works, and not how good it is. Every single frame in a digital video can be completely reconstructed, by using information that is in the video-file/-stream. H.264 will give better results than MJPEG, even when you export one frame, at the same bitrate. This is valid for most bitrates (not sure about very low bitrates). There is more information and several tests on line, and a good place to start is compression.ru.
The only disadvantage with H.264 is the computational complexity in reconstruct a single frame. H.264 requires a lot more processing power than MJPEG. This is not a problem on a modern computers though.