Because you dont know how zoneminder works. If it were to record everything 24/7 blindly.. yes you are right almost zero processor used.. Zoneminder looks at every frame doing motion zones and detection. It's 1000% better than the best commercial DVR you can buy. Really it is. I have tried all them including the high dollar ones, Zoneminder kicks their butts hard.
Sorry, but your completely wrong. The idea of motion detection on digital video recorders is over 10 years old. All DVRs of any value have motion detection built in, and the BT878 MJPEG cards mostly used with Zoneminder are garbage. The MJPEG codec is the wrong one to use for most applications because of the high storage requirements. Yes, you can process motion detection faster, but you loose out on bandwidth and storage. Zoneminder can use IP cameras, so that is a plus in its favor.
Also, your point about no CPU usage at full 24/7 is also incorrect. The BT878 cards require software compression, which means CPU overheard just to lay down the video. There are other cards, such as from Vidicon, which allow for hardware compression thus offloading the bulk of th CPU requirements.
Again this is nice, but very home-brew type of solution. If you are a professional or just a larger business who takes into account the man hours required to build, and most importantly to maintain this system, you should go to one of the "commercial DVR systems". These will give you the required reliability and low maintenance, in addition to a smooth interface usable by more then the IT guy.
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