Studies of rappel forces, sponsored by the AAC, concluded that typical rappels could easily generate forces equal to twice the weight of the rappeler. You are probably wearing your pack, and odd bits of gear and clothing as well. Most climbers want a 5:1 or 7:1 safety ratio, at a minimum (at least those who have survived to discuss their practices).

There is also the question of abrasion and wear of the rope, which can happen very rapidly, reaching dangerous proportions even during the duration of a single rappel.

I would really prefer to carry a thicker rope - the extra weight is well worth the increased weight.