I absolutely LOVE having a travel system. For me it has given me brilliant flexibility to be able to go out and do what I need to with minimum disruption to my bubs. Why wake up a sleeping baby to transfer in and out of the car?
I have a maxi cosi mico and mammas and pappas sola frame which means I have the option of using as a stroller which is suitable from birth to toddler, or using the maxi cosi car seat clipped into the frame which I am still using and he is 6 mo (obviously the length of use depends on the size of your baby).
When he was a newborn I would pop him in just before he was due for a sleep and then be able to go out and not worry about disturbing him. I could get him nice and comfy in the comfort of our home, and if he was still asleep when we got back bring him back inside until he was ready to wake up. A short trip to the shops is difficult enough when you first have a baby, so for me the travel system made it much easier.
The first time I attended mothers group loads of people had the bassinets and many commented on how great my travel system was and wished they had one.
Of course, if you don't think you will use the car much then it isn't worth it, so this does depend on where you live.
Oh and about it "not being good for their back", as long as you don't have them in it for more than a few hours then I don't see why it is a problem. It's no different to having a baby in a swing or bouncer chair and many people sleep their babies in those.