I've got about a 10 micro-pc and embedded projects around the house.
Five of those projects use an arduino (usually a nano) because that's all they require. They are cheap, reliable and dead easy to program.
The remaining five use RPI's. Three of those are small headless servers (file server, zoneminder and squid proxy), so they need Ethernet for the performance. The fourth is an OctoPi 3D Print Server, it only needs USB Serial and USB Wifi. The last one is a Brew Plant controller running a webserver, it's on a mobile platform inside a sealed box so uses USB Wifi too (and the GPIO).
So out of ten projects, two of those could have used a much cheaper PI Zero with absolutely no loss in functionality or utility, and the reduction in size would have been an advantage. Every project has a different set of requirements, sometimes there is overlap between products and sometimes it is clear. One thing is for sure though, there is definitely a use case for the Pi Zero,