Integrated Child Safety Seats
Ah, yep, I've been trying to figure out who it was I know with a late-90s Voyager to ask about their built-in seats ... some vehicles only have flip-down boosters without harness meant only to lift kiddo up for use with regulat lap/shoulder belt & that's what I thought you meant
If yours has the 5-point harness built in then that's great!
Very nice for picking up kiddo's friends & there's no worry of incorrect installation, but I myself prefer to use a separate carseat (assuming proper installation) because in the event of an emergency kiddo will need a separate carseat to travel in another vehicle & a loose carseat in the vehicle is a potential projectile during crash
Also, more & more carseats now keep kiddo harnessed past the minimum 40 pounds with added comfort features that can make long distance travels safer & more enjoyable for the whole family, but it's common that such seats need to be saved up for because they are rather expensive & you've already got 2 good seats that will keep your kiddo safe now.
Carseats generally only have a 6 year lifespan, but I've been told that the Integrated Carseats like yours have the same lifespan as the vehicle seatbelts: which simply means that if the integrated harness seat is occupied during a crash it must be replaced, but otherwise has a very long life & would be safe to use....
So, if it were me, if kiddo doesn't have any problem with his current seat I'd use it & leave the Integrated Carseat space open just in case any other kiddo rides with us who might fit in it -- this way all bases are covered
If kiddo wants to ride with someone else you've got a seat to put in their car & still have a seat for him to use in yours if by chance the other carseat isn't available.
Or ... Does kiddo regularly ride with another family member or care providor? If so, then you could consider having the carseat you showed above installed in that vehicle & leave it installed (checked by a CPST) -- this would prevent repeated uninstall/reinstall which can lead to less secure installation -- having the seat more or less "permanently" kept correctly in the 2nd most used vehicle that doesn't have Integrated Carseats could be a great idea ... although if you had car troubles you'd have to call that person up for a ride since kiddo's only spare would still be in their car, not yours. It's for this reason that we had various carseats: in my car, in the babysitter's, in Grandma's, etc. & I didn't have to worry about risking improper installation!
Just always remember to follow the manuals & feel free to call the manufacturers with questions!
I think there's some info in the FAQ section of this site above on Integrated seats, too?
P.S. sorry I think I said "she" earlier when he's a he, LOL!