No, you don't NEED it but I wouldn't have gone without. Of course, my baby was small so she used it for a long time. The combo of the infant seat and a snap n go was my favorite way to get around. I loved babywearing, too, but there were times where I just wanted to put her down for a bit and the carrier/snap n go allowed me to do that. Then if she fell asleep in the carrier I could just pop her back in the car. My kid was fussy and a crappy sleeper so I NEVER woke her if I could avoid it. Many times I brought her in from the car in her carrier so she could nap for a few more minutes within eyesight.
Also, dropping her off at the babysitter was so much easier when I could just pop the seat out and take her in that way. Also, my DH and mom had bases as well so then it was easy for either of them to come and pick her up without everyone having to have a huge convertible seat in their car. Of course, now that she's 2 they have big seats installed but it was nice to postpone that for a while.