I travel light. I have since the 90s. I never ever ever checked a bag before 9/11, but since then I've often done so for two reasons:
- Absurd restrictions on liquids in the cabin; and
- Equally absurd restrictions on carrying my Swiss Army Knife.
Once you check a bag, you need another bag to carry some essentials aboard the plane.
Even on a short weekend trip, though, if I buy sample sizes and ditch the SAK, I end up with a small additional bag. It's easier to do that than constantly open the overhead to get a different book, or your tablet, or whatever.
One-bagging is also a poor choice for business travel, since you likely want a smaller computer bag so you don't have to use your much larger "one bag" as your office bag.
OneBag purists also have a WILD hatred of wheelie bags that seems rooted in a refusal to realize that, for many people whose travel patterns are home to airport to destination hotel/home, they're perfect. The extra weight isn't a problem because they roll. There's a great reason why they're very very popular with full-time/very frequent travelers.