[BrianG]

Serum is right: I was looking at it from a service perspective. I also like the reduced thermal resistance to heat by heatsinking the metal tab (which conducts heat far better than plastic I'm sure you'll agree). Heatsinking the plastic case does work, it's just not as efficient.

And you are right about FETs: as they heat up, they conduct less (negative temperature coefficient). However, this is more of a circuit design parameter to allow them to be paralleled without one or more "current hogging" as compared to a traditional bipolar transistor. As far as high currents are concerned, FETs can still burn up. Any package type that does not shed it's heat will eventually melt. Some of the problem here is the use of the PCB to act as a heatsink and to thermally couple the layers of FETs together so they run at the same temperature. It's just not enough apparently.