I have a Moby wrap, ring sling & an Ergo (RS & Ergo bought used; Moby was a gift). I use the Moby & RS for wearing in the house, and the Moby & Ergo for shorter trips outside.
We were gifted a BOB Revolution Flex stroller by my in-laws. Yes it's expensive. Yes it's big (but the only stroller we demo'd that easily fit in our 2003 Cavalier trunk).
BUT
I'm now 8+ weeks postpartum & take the kid and dog on daily 1+ mile walks. The stroller handles snowy sidewalks & muddy paths like a dream. We're 1.6kms from the grocery store, so I can easily walk there, get food & back-haul. Once the kid is a bit bigger (~6 months, with good neck & trunk strength) then I can take her jogging with me. If my plans were to drive to the mall & walk around then a cheaper stroller with dinky wheels would be a-okay. But that's not our life, so a bigger/more expensive stroller is what works best for our situation. Is it absolutely necessary? Hard to say, but I wouldn't go outside nearly as much without it, and I think being outside/active has contributed to staying healthy both physically & mentally (VERY important in the postpartum period).
(PS: I haven't yet tried to take the stroller on the bus. If/when I do it'll only be @ non-rush hour times because, as another posted said, I don't wanna get stabbed for having a large stroller on a packed bus!)