If a phone isn’t durable/grippy enough to survive dropping and being used without a case, so you’re going to be adding a big chunk of whatever, why not put the entire battery in the case?
Then you can buy a slightly thicker one for longer life. Way cleaner/lighter/thinner than the the terrible battery cases that exist now. Year old phone isn’t lasting as long as it used to? Buy a new case and you get a fresh 0-cycle battery.