“major custom css” is not a prerequisite for using custom.css. In fact, I would say that using a custom.css file makes the modifications you DO make much much easier – to find and re-modify when/if needed. It’s a “one-stop-shop” per se … as opposed to having to try to find the right CSS file/rule again, and again, and again. 😉
ALSO, using a custom.css file protects your customizations/modifications from future updates – as there are not any ‘custom.css’ files within any updates – therefore, nothing gets overwritten. The same cannot always be said for core CSS template files.