Save yourself an extra luggage fee and opt for one well-packed carry-on. Use that extra $15 to $25 for something special you find on your travels.
Lighten your load and experience the freedom. Travel (especially air travel) can be cumbersome. Hauling more than one piece of luggage around the airport is anything but relaxing.
Stretch your traveling prowess by packing and journeying light. Packing might not be the most exciting part of your trip, but mastering the art is a fun and creative way to boost your brain power and self-discipline.
Go more places with less stuff. Backpackers see more things if they don't have to worry about transporting a lot of baggage. A single backpack with the essentials will allow you more movement.
Make yourself a smaller target for thieves. If you are lugging around several large bags, you become an instant mark for thieves. Keep important items close to you in a small unassuming bag or backpack, and leave all items of great value at home.
Pack less, shop more. If you are a shopaholic, the idea of leaving certain items at home can be appealing because it allows you to purchase them on your trip. Coming home with a new dress from a fancy boutique you discovered can be the best reward for not bringing your "little black dress" from home.
Be ready to go when the opportunity knocks. If you can pack fast and light, then you can be ready to travel at a moment's notice. Knowing how to pack and travel with one bag allows you many more chances to see and explore the world.