We had both, an infant carseat someone gave us and a convertable carseat. The infant carseat was good for the hospital because they insisted we showed them the carseat properly adjusted in the hospital before we left. It also fit into our honda civic very well and it was nice to be able to bring it inside if the baby fell asleep in the car on the ride home, but we never used a stroller, so if we went out somewhere I put my daughter into a baby carrier anyways. The issue we had with the convertable carseat was that it did not properly fit into our car in the rear facing position, it didn't tilt back enough and it was big on my daughter even though it claimed it could be used from 5 pounds up and my daughter was 6 pounds and taller then the minium as well. She didn't fit into the convertable carseat well until 3 months old and we didn't use it full time until she was 7 months old and the bucket carseat was on it's biggest setting, once again even though my daughter was below the weight and height limit. We have since also bought a affordable 2-1, infant and toddler (not infant, toddler then booster like the other convertable carseat) and I think that may have worked better when she was an infant. Another good thing about the 2 in 1 is that it's more lightweight then the other convertable carseat and we have carried it inside with my daughter in it asleep without waking her up (even up stairs!).
So basically, yes I think you can get away with not having a bucket carseat and just using a convertable carseat if it is the right one, that fits the baby and fits into the car you have.