And now for historical commentary: As equill points out, cost is about the only factor about sizing up. In Larry Pina's excellent book of Macintosh Repair he really goes on a tirade about Apple's choice to use low temp and undersized caps in the original Macintosh designs, a choice they clearly made based on analogue board designs which accommodated larger more robust parts as well (thank gods for engineers). He states they got it right by the final run of the Plus, but even still a few of the caps were undersized, which he recommends upgrading. He blames this more than anything on the high failure rates of the original Macintosh, not the fan-less design. Tom Lee's excellent paper summarizes much of this discussion as well.

It's like some sort of greedy profit strategy:

Cost savings from use of inferior parts, minus

Savings lost to warranty repairs, plus

Profits earned from Apple Care, minus

Profits lost to warranty repairs, minus

Cost to minimally upgrade high failure parts, plus

Savings from fewer warranty repairs as the newer parts last just past the warranty period ends, equals:

more profit than if they'd used the proper parts to begin with ...

...otherwise why do it that way, when they clearly knew better?