I believe the standard is under 12 months/20 pounds for rear facing car seats. It certainly is possible to install a bucket seat facing to the rear without the base, but it is tricker to get it at the right angle to support the baby's neck properly. One option is to use a rolled up towel under the edge of the seat closest to the car's seat, but you might not be lugging one of those along.

It comes down to what you think is easier - lugging the base along, or re-installing the seat every time you get in the car. Based on my experience, I think it's easier to do a good job installing the base if you take time to really tighten the seat belt down than it is to get just the bucket safely tightened down every single time.

FWIW, the airlines always allowed us to check the seat base without counting it against our luggage allowance. We just wrote "infant car seat" on the box.