I'm going to guess - and that's what it is - a GUESS - that the resistor pack is a marginal design that didn't account for a wider variation of motor current draw. It would be expected that an otherwise operating motor would tend to draw slightly - emphasis on slightly - more current as it gets older, esp. if it has sub-standard bearings. *shrug* Yea, if its melting wires and control knobs, that's one thing, but if its burning up resistor packs "occasionally" that's another.
My jeep had its r-pack replaced just before I took delivery of it. Was that the original pack that had failed? Who knows. A friend had a TJ back in the day, and he replaced the r-packs so often that he carried a spare in the glove box and had the procedure down to 30 seconds or less.
If this problem is as common as I've heard it is, its indicative of a fundamental design problem, be it with the motor, R-pack or both. As a brand new Jeep owner, I'm discovering that there are several sub-optimal design issues - particularly with the radiator and heater core. I don't like fixing things twice.