I started dual deploy in 1999. I never had a barometric altimeter fail to do its job for deployment. I have 3 MAWDs still going strong, even after a couple water dunkings. Also Missileworks, MARSA, and Raven. I ground test. Any failure was due to me, and I can count the incidents on one hand - loose battery connection, crappy homemade ematch, bad parachute wrap. I worry more about ematches these days, so I occasionally using 2 per charge or 4-channel alts if I am feeling up to it.
So, I am curious as to what started the multiple altimeter craze. Was there a spate of rockets augering into the ground with one altimeter in 1999? I don't think so. My guess is that price, not need, is when the redundancy started. Below $100, altimeters started to look like a bargain for most folks, and the usual over-building mindset sunk in.
I don't have stat data, but the baro altimeters seem uber-reliable. Add another altimeter, and the build complexity goes up along with human error. So, to you redundanites, what are you protecting against - the altimeter or yourself?