Well, it does help to open those cases, it may not show that eBay is taking any action it's not obvious on a seller's account, but if too many cases are opened, resulting in above average refunds in percentage number to the number of sales, they will take action. They'll serve selling sanctions or even bans from selling.
eBay don't like disappointed buyers, they tend not to come back, so yes, eBay do take action against bad sellers, but only if buyers take the time to open those cases.
@simon888cat