I love my backpack. It enables me to carry a lot of stuff with me wherever I go and I can keep my hands free while I walk. It's an everyday backpack (not some mammoth hiking lover's thing), and it serves the same function as a purse does for others. I realize that wearing a backpack probably fixes me as the real-life caricature of your beer-loving, slack-jawed thirty-something graduate student who is "still working on my dissertation." (Cf. Bart Simpson's disturbingly apt take on the lifestyle.) But as long as it is professionally feasible and I am physically able, I'll continue to wear it.
The "physically able" part might, someday, be a challenge. A certain professor (probably in his 60s) I know and who I've TA'd for a couple times once commented on all the students who wear backpacks that they're taking their ability to wear backpacks for granted. This professor has back pains that prevent him from carrying anything too heavy, and he couldn't probably sport a backpack if he wanted to.
I'm not there yet. In fact, I have always (knock on wood) been physically healthy for the most part. But I can say that when I (at 37 years old) wear my backpack now, I feel it more than I used to. It doesn't hurt, and it's not too heavy, but I do feel it. I'm more aware of wearing it. As much as I like my backpack, I sometimes have a brief thought of "wouldn't it be nice not to have to put it on?"