Josh Gerben was quoted by Corporate Counsel, saying:
Use of the symbols may serve as a deterrent for would-be infringers, said Josh Gerben, principal at Gerben Law Firm. “Most of the time, you get this self-policing mechanism out of using the trademark symbols,” he said. “They will typically steer people away from using those trademarks when they know it’s the property of someone else.”
“You put it on every time because you hope if something happens, you have the protection you need,” he said. “Is it a 100 percent required to protect the marks? No … but it just adds more cards in your favor.”
Article: Williams-Alvarez, J. “Trademark Symbols: Powerful Tools for In-House Counsel?”. Corporate Counsel. July 10 2017.