Another rather unpleasant thought is that one reason designer fuses blow when they shouldn't is that the makers of such fuses have one goal in mind, to maximize profits. That would entail buying the cheapest fuses from the cheapest manufactures ($.05 each?), adding their "magic" touches, and then selling them for the most profit possible. Having to replace a few of the really defective ones, no biggie.
Never underestimate the profit motive.
Never underestimate the profit motive.