Are there any advantages or disadvantages for a child in just a seatbelt being put next to a car seat? S passes the 5-step test and uses just the seatbelt. Z is going into a booster this summer and will be outboard as well (lapbelt in middle). I will be transporting a three year old in either the GN or Scenera a twice a week when I do not have S and Z. Both install easily in my car, so I don't mind taking it in and out, but would it provide any safety benefit to S to have it permanently installed in the middle? I was thinking in terms of a side impact it provides less room for movement, but it is also a hard chunk of plastic so I don't know if that would be any better.