That's a BIG mistake, we just took a big hit in the rear end, I think the Evenflo Triumph seat combined with Isofix/Latch system is what kept my daughter without any effects. Why would you not want to use it? I don't think there is any way a seatbelt setup will fasten the seat down nearly as well. MINI has made it a really easy system to use, unlike all the others where you have to dig around between the cushions to find where to latch it. With MINI just flip open the covers and there they are. Don't forget to use the top tether either. And when tightening down the front straps get in the seat pressing down with your knee and LOTS of force and cinch the strap up good and tight.