The advantage of something such as the MET5 should be pretty obvious. All other insulation out there is passivethat is, it traps body heat, but does nothing to generate heat and can be overwhelmed if conditions turn too cold for the weight of the insulation. Moreover, most insulating materials are not terribly dynamic and can't change their essential functions on the fly. So in stop-and-start sports such as downhill skiing or ice climbing, you can find yourself sweating heavily while you're working hard (running moguls, climbing a pitch) then freezing when you're not (sitting on the chairlift, sitting at belay). The MET5 circumvents both these problems, generating warmth when you need it, in the output that you need.
Of course, its downside is equally obvious. It requires batteries, which have a pesky habit of running down at inopportune times. Hence your suggestion to make it solar-powered. Which I like! And certainly, solar power has proven itself useful in plenty of applications. In addition, portable solar-charger units work well, aren't terribly expensive, and don't weigh much. So why not integrate one into a jacket?
Well, that's a gauntlet I'll pass over to the gear makers. In part, there are practical issues at play, such as the fact that such a garment would require that the weather be cold but also sunny, a combination that isn't available, A) when it's cloudy, and B) at night. Plus, the tepid sales of the MET5 indicate people aren't quite yet ready to rely on wires and batteries to keep them warm. Easier, and cheaper, just to buy another fleece jacket.
Check out more jackets, soft shells, and fleeces in Outside's 2004 Buyer's Guide.