There are a lot of reasons we say 8 for backless... it's my comfort-zone age for it, too... and I've HAD to put both my boys in backless boosters well before that age... with Jeffrey, I was able to go back to high back because we got him a Monterey... my boys are tall (especially Damian who was 4'9" shortly before he was 8 and passed the 5-step test at 8 and is now 5'3" and 115lbs)... and Jeffrey has one heck of a torso.
Anyway... back to the topic... the reason 8 is the comfort zone....
1. Maturity... somewhere around age 7-9, kids get mature enough to stay in position all the time even without a back on the booster... so just say the middle/average of age 8... the rest also depends on size of the child...
2. Age - even the states with the strictest laws have 8 as a minumum (except for Washington, of course... how I love it) for using just a seatbelt... and even with that in place, many of our kids' friends are using just a seatbelt before that... and at sizes they should still be in a booster. So... a child may be feeling peer pressure... and you really have to pick an age where you're okay letting a child have a say... 8 is when I'm comfortable... sure, the 7yr old or 7.5yr old or 7.75yr old has just as valid a concern... but I would still say... when you're 8 unless you get too tall for your booster's back.
At age 8, when lots of parents throw away the booster because they think the law is enough... I'm fine to compromise and say they can simply put away the back of the booster instead... After all, it doesn't look as much like a carseat with the back off.... and many times, no one can see or know it's there unless they are sitting in the car with you.
3. Other factors related to age... being able to stay in position when asleep... my oldest, who ironically had a mild scoliosis that had me put him back in a harnessed seat back when the only one that would accommodate him was the Regent... and then he outgrew that in 9months... was able to do this when he HAD to go backless by outgrowing his old-style parkway at age 7... the Monterey wasn't out yet and obviously wouldn't have been worth buying for him because it was released after he was passing the 5-step test...
Most kids, due to size partly (Damian managed it at first by tilting his head back against the vehicle seat's headrest, where it sort of held the top of his head by it being angled... before he was out of a booster, he could no longer do that.. and now he can't do that.... but he's mastered sleeping bolt upright... keeping his head tilted back slightly and to one side... resting it kind of on his shoulder.
Jeffrey is now 8.5yrs old... and he is okay at doing it without a headrest and back on his booster... fine enough I would let him be backless all the time if he wished... and he was for awhile after he turned 8 last summer and was feeling pressure to be in nothing... but on longer trips or when he's tired, he found he couldn't sleep soundly because of the lack of support... he prefers his highback booster.
But again.. he COULD stay in position the whole time without the back...
Anyway... that's me... my reasons at least... basically maturity and picking a median age of when all those things happen.
OH.. plus, we're talking about a spare seat situation where this would make it easier for the child to be totally independent in the setup and use of his seat in someone else's car... and that alone can help prevent friends from teasing...