Used one for a while, mainly because
1 Both my DC are big and heavy (both 98th centile weight and height), so the toddler in buggy/newborn in sling thing was too much for my back as both buggy and sling were v. heavy
2 In the winter it was useful being able to keep the older DC warm and dry in the buggy at times
3 Wanted to have the option for crowd control - if toddler kicked off in shops etc
As it was, we used it for about 8 months - not long, but I was very glad to have had it. We bought new so that we could have what we wanted (twin Nipper, fab machine) and looked after it well, then sold it.
I predict you'll have a real split in responses, 'oh no, no need' and 'oh absolutely my nearly 4 year old sometimes needs to go in the buggy' but ultimately it's about your toddler - how he/she is out and about, how strong your back is, and how much 'out-and-abouting' is involved.