Great topic.
I've been looking in to this myself, I need to add a cam to my trucks bumper for backing up to find the hitch and wanted to put a cam on the back of the trailer for backing it in to spaces. (and not running over stray kids when backing, lol)
Then I remembered something on TV, all the buses here are outfitted with cams. (they get hit often) and was thinking of getting a mini security DVR which constantly records. (they are 12v after all, as are the cams.) We get a lot of tourists here, who seem to have broken turn signals and brake lights. Needless to say, there are a lot of accidents daily in the Disney heck area. (but we do see free fireworks everynight, just 10 miles away.)
I'd add a forward facing cam behind the mirror and let the DVR record just in case. (video is the best evidence of what happens on the road, lol) I didn't think of adding some on either side for blind spot (until this topic), but thats a simple matter connect the autoswitch to the turnsignal...when the signal goes on, the monitor shows that side.
Is it worth it, prob not... Would I use it, maybe only for backing which is the purpose. I like the blind spot idea, just incase it's a bad angle and the mirror can't see it. But I do like the idea of having a recording, espessialy if hit by a tourist with no insurance. (locals pay double to have un-insured insurance) Or if an accident happens in front me. It's toy, but so are heated seats and a radio.