To put it another way: every team has a ceiling. If you start with 2 optimal SSR kamihime picks, you're going to hit that ceiling sooner rather than later. And then what? You'll get bored.

A +100% eidolon raises the ceiling significantly, giving your team more room to grow. Rather than 2 more kamihime, you'll likely need all 4. If your goal is to use one team for all content, there's simply no substitute for a +100% eidolon.

And if you're a free player, you'll probably have more fun seeing your jewel rolls actually improve your team rather than starting with a team that is nearly at full potential and where you'll only see incremental improvements.