I intend to harness my child as long as I can.
I hesitate to recommend that course of action broadly, because I've seen first hand how difficult it is for even the most conscientious parents to use car seats correctly every time. I also run across a lot of parents who don't realize their seats have *height* limitations as well as weight ones.
Most children will outgrow even the mid-range higher-capacity seats (Secure Kid, etc.) by height at around age 5 or 6, so that's usually the time I tell parents they should be moving to a booster. I find that instructions like "tops of ears level with the top of the car seat" or "shoulders over the harness in its highest position" are just too complicated for most parents to keep in mind. Car seats are not the biggest thing on most parents' minds. They want their kids to be safe, yes, but the day to day obsession that CSO parents like us have with car seats is not normal. So "Most kids are ready for boosters sometime between the start of Kindergarten and the start of 1st grade" is a tangible, easy to understand concept that is more likely to "stick." NHTSA's recommendations of "harness as long as possible" leads people who consider themselves experts to post You Tube videos of their 9 year olds in Recaro seats with shoulders about 4" above the harness... just like the "rear face as long as possible" sometimes leads parents to drive around with their 34 lb children in SR35s. So instead of just telling parents to RF as long as possible, I tell them to use the infant seat until about age _____ (depending on which infant seat they have), get a rear-facing convertible, and come back for further instruction. Yes, I go over more details than that, but I don't expect the parents to actually retain 30 minutes worth of car seat check material. I repeat something like this over and over: "This seat should last until about 9 months, maybe a year if your child stays short. The next step will be a convertible seat, installed rear facing. Please come back when it's time to make the switch so we can help you again."
So, yeah. I'm an advocate of extended harnessing. But I am careful about how I convey that to parents. I can't count the number of people I've known (the ones, usually, who think they know too much to come to a car seat clinic unfortunately) who brag about keeping their 8 year olds in harnesses and who are totally oblivious to the fact that the seat is outgrown, loose, and not tethered.
Me? I am not at all convinced that neck loads are as much of a concern as head excursion. If they were, then we wouldn't be top tethering harnessed seats. I would wear a 5-point harness in the car if I could.