if the belt fits properly, then it's fine to use one as a spare (or as a primary seat if a highback isn't compatible with the vehicle or isn't able to be afforded) provided the child doesn't prove too impulsive without the back.
obviously, in your (OP's) case, you've decided it's not a good option for you in your situation.
As far as size... well, Damian was just too tall for the OS Parkway (tallest booster at the time) at 6.5, which is the only reason he went backless full time... (and outgrowing his turbo when he outgrew the regent meant he was already backless as a spare seat because of necessity)... I hated having him backless that young... just looked so vulnerable... even at so big.
but because he was able to stay completely upright even when sleeping (for the main seat) I didn't get an 86-Y (I would have if he'd flopped)... and why I didn't buy him a SK Monterey (I would have scrimped and saved and made it happen) when they came out... even though he didn't have much grow room, I would have if needed.
Jeffrey, I was OKAY with him backless once he was 7-ish (a bit older) because he had also already outgrown the turbobooster... at 8, I was fine with him using one full time... but even though he'd stay in position sleeping, he voiced he wasn't comfortable that way and so he and I preferred him to continue use with a highback...
so I guess in terms of size... I want about a 19-20" torso (outgrown turbobooster, evenflo big kid, other boosters of that size torso) and the boys were over 60lbs.
For Ruthie, she's more petite... at 4' tall and 53lbs (and I won't be surprised if she drops back down this year, which she did last year, when she had gotten up to 55lbs and then dropped down to 47lbs during an illness) She does not get a good belt fit from most backless boosters yet, because of her narrow shoulders... so I'm not putting her in a backless until I have to... even though I know she'd sit right.
that will be in 2013 (when her vivo expires... unless her torso shoots up about 3-4" first, but I doubt that... she's dropped from the 50 to the 45th percentile for height)... I don't know how big she'll be, but I highly doubt she'll be remotely as heavy as her brothers were when they went backless... but I know she won't drop down to under 45lbs, so that's fine... I just want to give her time for her shoulders to get wider and ensure a proper belt fit.
She's all of a sudden not the car sleeper she used to be, but she still leans her head against the headrest when she gets tired and when, in the past, she would have fallen asleep, so I think she would prefer staying in the highback still, even if she had a choice.
before, I thought I'd let her go to one as a spare when she turns 8... but now seeing how small she still is, I'm not okay with it and so I'm hoping she'll be big enough when she turns 9 in another year and a half.