I'd say it depends on the kit. I recently built the Tamiya Nashorn 1/35 scale and you couldn't find a need for any PE, or aftermarket, parts on that kit even if you tried (apart from perhaps a metal barrel?), but I've built a couple of kits that could really need some PE parts to better represent thinner, and more detailed, parts. I'm not building tiny scale kits so I can't comment on those, but I think it's up to each and every modeller what the kit means to him/her and how far he/she wants to take the build?
I've seen pretty wonky kits being painted, and weathered, to look absolutely stunning without aftermarket parts so it can be done, but some things are just easier with PE parts in my humble opinion, like grilles e.g.
I'm learning to make my own PE parts and I've got the process worked out with brass sheets. The design part is what takes a lot of time though... Many hours of CAD work and then making sure it fits...
Not for everyone but a simple process for us who regularly etch PCB's at home.