How often do AVR's actually glitch and need a watch-dog reset in the real-world?
Cosmic ray hits and SEU (Single event upsets) are very real.  Just look up data about DRAM and the need for ECC (Error correcting) and from that you should be able to get a sense for the probability vs. area.  Some processes are less prone, and smaller processes while being more sensitive also present a smaller capture cross section, sometimes that is a benefit and sometimes not.

Keep those watch dogs running!