Without © in Your Work, Infringers Can Claim They Didn't Know It Was Protected

Not true. "can claim" means nothing and "did not know" is not valid defense. Copyright is well protected without any symbols or copyright notice. Content is automatically protected by copyright. With, or without copyright mark you need to fight the one who uses your content without permission.

If you want to make claiming copyright easier, register your unpublished or published work. In US you can do it in U.S. Copyright Office

In EU you can use Datasure in Copyright.eu to get official proof of the date and content of your creations.