Which Chain? (Chains / Size / Width / Wear)

Tbh on cheaper bikes I tend to just join the chain normally, i mean, with a fixed gear you're going to have to cut the chain down anyhow, why not just rejoin it old school as you are going to have the tool?

Because the best ⅛" chains have heavily peened pins, which are not reusable. Sure, on cheap flush riveted chains you can do it the old fashioned way with a modicum of success, but it will always be inferior to doing it properly.

Also, very few dérailleur geared bikes need the full 114-116 links that come in the box with dérailleur chain, so your argument would apply to them just as much as it does to single speeds