I agree, bring your own! I worked for a major rental car company for years. I can tell you the seats are gross! They are also thrown around alot which could affect the safety of them.
Also, if you are flying, you can use them on the plane. It's safer for the child to be in one. (We always purchase a seat for our kids. Besides safety reasons, it's easier to keep them contained and it's familiar to them) If you don't want to purchase a seat and if the plane is not full, the flight attendants will probably let you use them. If not you can gate check them like the strollers.