I hope I don't get flamed by your guys but what if I use both?
I mean using tether with the latch along with the auto's seat belt as back up?
Car seats aren't tested this way, so in essence installing a carseat with both the lower anchors and the vehicle seatbelt is turning your child into a crash test dummy. Most carseat manuals specifically warn against using both to install a carseat - going against the manual's instructions also voids all liability the carseat manufacturer could have if the seat failed.
Seatbelt failure is so incredibly rare, but if you're worried, then you can always move the seat to an outboard position and install it with the lower anchors and just don't use the seatbelt.
I did a check shortly after becoming certified and the carseat was installed using both LATCH and seatbelt because they'd been unable to get a tight installation with just the one. When I unbuckled the seatbelt, the carseat was incredibly loose. In all likelihood it would've worked the other way too if I'd removed the lower anchors first. Another thing that happened, was the harness had been harder for them to adjust when the seat was installed with both - they didn't even realize it was harder until the seat was installed with only the one method and they adjusted the harness.
There are other issues too - I'm sure you could find a whole bunch of threads if you did a search. It's a question that comes up fairly often.
And as a general rule, we don't flame people around here. The techs and advocates around here are really only interested in helping educate and answer questions that parents & caregivers have. You'll find most discussions offer facts and recommendations, above and beyond that it's up to the parent. We all like to see parents make the safeST choice, but it's ultimately their decision and isn't for us to judge. :thumbsup:
FWIW, a seat being installed with both the lower anchors and vehicle seatbelt is probably one of the most common misuses when it comes to LATCH - the first one being borrowing anchors in vehicles that don't allow it. So you're certainly not alone in wondering if one would be good back up for the other. But until a manufacturer starts telling us to use both when installing a seat, I'd leave the testing up to them and just choose one or the other.