My personal view is that altimeter redundancy should be practiced in all rockets unless there is a really good reason such as (1) it cannot fit or (2) there is something really special about the flight like it is an altitude record attempt and the mass/space reduction is worth the risk, or perhaps (3) you simply cannot afford the extra $50 for a backup.
The reason for this is purely safety. I don't care if the rocket is inexpensive, small, or staying at a low/visible altitude. It can still make a hole in someone. Ballistic rocket scare the crap out of me. I had the pleasure of being about 400 feet away from a 3' minimum diameter K powered rocket come in ballistic. It makes you think. Small makes the rocket much more difficult to spot when it is coming in at the flight line ballistic. Low/visible altitude makes it more likely that it will land in a smaller radius containing the flight line. There will be many persons on the flight line who will not have success at jumping up, locating the rocket, and moving out of the way when/if they hear an announcement that the rocket is coming in hot.
For general flights not violating the the three above principles, why not include redundancy? As others have pointed out above there remains a small percentage chance of a problem with an igniter or computer or intermittent wiring problem such as short or open-circuit. I personally also like to have a backup charge that is 25 to 50% larger than the primary charge in case the deployment is being hampered by friction.
For flights with main at apogee I often use a single flight computer configured for redundant apogee and a separate charge attached to both apogee and main. The computer is not redundant, but everything else is.
I do not like motor deployment as a backup (or primary). The delay time can have significant variability causing zipper/recovery separation by being either early or late. Lately I have been flying rockets where the time to apogee is much longer than available motor delays so it isn't even a consideration.
Here is a
link to a stock 38 mm diameter 6" long ebay in which I put fully redundant dual-deploy with GPS and independent batteries, without it being a real pain to use. It took some effort to build, but for me that is actually part of the joy of rocketry.