Just today I decided to try something that I was certain would go terribly wrong: In the Versioning Settings of one of my document libraries I set the Require Check Out option to "No" but left Document Version History set to "Create major versions".
I saved the change and started making test changes to documents I didn't mind losing if things went sideways. As far as I could tell everything worked exactly as I wanted it to; new versions were created and other users who tried to open a document I was currently editing were shown a popup informing them of that fact and offering to serve them a read-only copy. And best of all, I didn't have to worry about checking anything in or out.
Near as I can tell the only thing I've lost is the ability to easily see who is currently editing a specific document. That is a feature I can do without.
So what I'd really like to know is why on Earth would I want to require check out on any of my document libraries? What is the benefit?