I am committed to keeping dd in a 5-pt harness for as long as possible. I am planning to buy another one within a few months as dd is close to outgrowing her Roundabout.
BUT. We only have one car and my dh has it most of the time. There are frequently times when I need or would like to go places and could by using a cab or carpooling. These are short trips, usually less than 15 miles. We have a spare seat (Scenera) that we keep in the garage for this purpose and to use when we have an extra kid in our car. While we have used the extra seat in our own car several times, I have avoided using it in cabs and other's cars because I get nervous about installing it. I can install our seats fine in our own car because I know exactly how the seatbelts work, where the tether points are, etc..... Many of my friend's cars do not even have tether points. And do not even get me started on the cabs in this town.......they are older cars and often do not have tether points.
Dd is 4 yo and meets all of the criteria for proper booster use. She is also very concerned about safety and I do not envision any issue with getting her to sit properly. We have not gone to a booster in our own cars because I do feel a properly installed 5-pt harness is safer. Dd also likes to sleep on long trips which would be a no-no with a backless booster. However, I am starting to think that an improperly installed 5-pt has got to be less safe than a pretty-impossible-to-mis-install backless booster. Not to mention how much easier it would be to haul around a booster around the grocery store until the cab comes back to get us. Plus, I do not want a situation in which a friend goes out of their way to come and pick us up only to find that her car does not have tether points or I cannot get an install I am comfy with. Lastly, the ILs occasionally keep dd for multiple days and have been horrendous about making sure her seat is properly installed. At this point, we do not allow them to drive her even short distances due to this problem.
If you were in my situation, would you purchase an inexpensive backless booster for these rare and short, but sometimes unavoidable situations? Am I nuts to consider putting my small (yet still big enough) 4 yo in a booster occasionally? Is it starter to rely on the car's seatbelt with a backless booster than risk an improper install due to unfamiliarity with a car and little time to figure it out? We would never use it for any trip long enough for sleep to be an issue.