It is really easy to move a car seat from the parent's car to your car. They aren't welded in or anything. After 4 years old they only need a booster seat, which is even easier.
It's only easy to do so if you're transferring them on an infant seat. It's a huge pain in the butt to move a convertible car seat. You'd have to uninstall and rei-install the isofix base each time. The most convenient thing you can do is to install those isofix bases in both cars, which is not going to happen.
You and I must have very different ideas of what pain in the butt is. I consider a 5-min install for a handoff a long time. With for non isofixed infant seats, you could replace a seat in less than 30 seconds. Regardless, I love my babies so no amount of time spent on them is too high a cost to bear.
