My first guess at an explanation, assuming the first poster is correct about the panels and welds, would be that they haven't narrowed down their specifications of the panels and welds yet, not that they have purposely left the welds as the weakest portion.
Another guess would be that they are using identical panels in many locations and have thinner welds in some of them. But that is sort of the same thing as above, because they'd likely make different panels as production volume increased.
Could be, considering we're talking about Elon Musk. If you know the thing is going to get caught on video exploding, then might as well make a good show of it.
> would be that they haven't narrowed down their specifications of the panels and welds yet
Sure, it's a prototype. Maybe they'll revisit welds on the next one. Or maybe they'll figure out a way to make explosions look even more cool.