This same question was brought up in a law class I took recently. After about an hour of "discussion", our professor brought up his take for our founding fathers use of a representative republic.

His view was this: We, as the general public, tend to become emotionally attached to the issues going around us. If we all were to cast votes to create or decide leglislation, we would be biased by OUR perpectives. We would not be objective; we would only see how it would effect our lives, not the rest of America.

Our elected officials are "supposed" to look at how both their constituencies and the rest of the country will be effected.