A lot of people prefer sitting upright but it's starts to get uncomfortable for me after awhile. Forget about cycling for a minute. If somebody is sitting in a stool or on a bench with no backrest, they'll often "hunch over" and place their elbows on a bar or table to help support their upper body. That's what you're doing on a bike with a more forward position. You're using your arms to help support your upper body. Many people prefer this for longer distances. The other often stated advantage to drop bars comfort wise is that they give you three hand positions with varying degrees of "hunched forwardness". This allows you to rest certain supporting muscles in your back and arms and lets you change the angle of your neck.

For me one of the most comfortable bars is a tri-bar. They take some getting used to and I wouldn't recommend them for commuting because steering is a little dicey, but I can ride a long way in that position.