Thanks to all for your responses. I am glad to see that my post was able to generate so much discussion.
It seems that the answer I am looking for can be summed up in one word: Efficiency.
Judging by the responses, it seems to me that a direct comparison between mopeds and motorcycles isn't really valid. They are used for different reasons. Mopeds are generally used for quick, short-distance commutes where it would make more sense to operate vs a bulkier, costlier, and less economical motorcycle. Makes perfect sense- why use a 16 lb sledgehammer when a tack hammer will do a better job!
Well, if nobody minds, I'll stop by this site periodically to check things out once in a while. Take care everyone!