We just had an interesting discussion on this in another list... some advice from a trusted mentor & wheelbuilder:
Wheel strength is determined first by spoke tension. The wheel can only carry as much weight as the tension in the spokes that it is STANDING on. If the spokes on the bottom of the wheel are slack, your wheel falls apart. Your bike wheel is NOT hanging on the spokes, it is standing on them. In fact almost all spokes break because they are not tight enough. The nipple and the rim will fail from over-tension long before the spoke breaks.
Having more crosses in your spoke pattern spreads the work among a larger number of spokes (and therefore the total tension available to keep the wheel together).
Tandems use more spokes for several reasons. More spokes mean that you can cross them more. A 48-spoke wheel can go five-cross. So the number of spokes that take the load can add up to nearly half the wheel! 36-spoke wheel can be built up to 4-cross, so it holds a heavy load. 32-spoke wheels are limited to 3-cross.
Trying to overload a rim and over-tension can lead to catastrophic failure of the wheel. So, more spokes makes more sense.