The answer depends on the number of people travelling with you and the age of the infant. If you have an extra seat for your infant, you are required to bring your child seat that you use in your car on board. This results in even more baggage. Also you can only be sitting at certain rows, since an additional oxygen mask is required.
If your child is still a baby (< 1 year) and you have at least 2 adults per baby, travelling without an extra seat makes perfect sense. You can shift and babies still sleep in the most uncomfortable situations quite comfortably.
If your child is between 1 and 2, I would recommend buying an extra seat. I am talking from own experience. We didn't in our last journey, because of the hassle with carrying the car seat. We really, really regretted it, afterwards. When our daughters (twins) finally fell asleep the next row decided she would too, and by reclining the seat waking our daughter.
Another source you want to check is seatguru.org. Here you can find the distance between the seats on major airlines. Some airlines still have a decent pitch to travel with a baby on your lap.