Do you think this is a good "emergency use" booster? The other day, at the last minute my sister called for me to pick up my 5 year old nephew from school. He is 55 lbs or so, but since this was an unplanned pick up by me I had no spare seat for him. My sister and I live within a few houses of one another, and I was thinking that for $15 this would be a good seat just to keep in my garage for cases like this. She has a key, so if for some reason she had to pick up my 3.5 year old unexpectedly she too could just grab this seat. I don't love the idea of a backless booster, but this seems like a decent option for situations like this.