There are -in my opinion- other reasons for automotive parts -eg: maintenance issues-. For a pure resistive load -fairies world with unicorns running around - the worst case scenario: happy mosfet. This is neither real nor enduring. Nightmares occur -using a PWM with FAN load its a good example-, and if you filled the block with epoxy -who said replace the damaged part ONLY ?- murphy said if anything can go wrong it will go wrong. The repetitive changes for inductive loads (real world: aka all loads more or less) it's the worst case scenario for a power mosfet. The ruggedness comes from a part that assure you that you can put the mosfet on its limits without damage ( working only with "Absolute Maximum Rating" values usually comes to parts dying or degraded performance over the time ). NXP had some APPs notes about this, pulse and overshoot models: http://www.nxp.com/documents/application_note/AN10273.pdf If I'm wrong, please correct me (I love when I'm wrong and learnzt things). I think the whole problem comes from the electromigration and damage to the insulation layer(s) which occurs on the die when induced the worst conditions which usually ending badly if the mosfet die with a short drain-source (you can turn off the thing and so on...). it's uncommon to see an inmmediate change -like a cliff-, it's more like a down road... to hell -and fire-. The first rule about the fuses: you have a PCB to protect them. How many roasted PCBs with intact fuses ?. The AEC compliant parts, and specially the above mosfet with automotive grade comes with greater insulation layer -i hope - and some internally tricks that makes this things more reliable. When all manufacturers have a specific automotive grade costing twice, not worth the money when replace whole -unfixeable- part that worth hundred times more.