I voted "other" because my daughter is in a booster full-time now, although she still has just a smidge of height room in the Regent (the only harnessed seat on the market she'd still fit in at this point). She's 7yo, always been large for her age (95th+ percentile). Harnessing beyond 40# is very important to me if the child is one who hits 40# at 2-3yo, and moderately important for kiddos who hit 40# at the more average 4-5yo, less important to me for kiddos who hit 40# at 6+. My reasoning is that being mature enough to sit properly in a booster is the key factor in a booster being a safe choice or not, and most 2-3yos are just not good enough at controlling their impulses to sit still in a booster all the time. Even many 4-5yos have trouble with this. The older a child is, the better impulse control they have.
My daughter started using a booster part-time at 4, but remained harnessed the majority of the time until right when she turned 6 (and about 50 pounds). At 6.5 we got a Regent to re-harness her, but then downsized to a Focus.
She's really, truly mature enough to use a booster properly, though, so we sold the Regent and she's using a Britax Parkway 99% of the time now, at 7yo, 53 inches tall and 62 pounds. The other 1% of the time she's using a Britax Bodyguard or Britax Starriser/Comfy (when we need to fit three-across). She's also a deep sleeper (saw that mentioned above) and a bedwetter, still, in
2nd grade, but in her Parkway and Bodyguard, we've never had an issue with head slump when sleeping (thank goodness), even on the rural, twisty highway I have to travel to see family. It's possible if she had a sibling, she'd be more likely to get out of position in the car, but she's an only, and I never have to remind her to sit properly anymore.