It's absolutely true that yard signs add to the visibility of the listing agent. And, more listings come from yard signs, as sellers see them and believe that the agent must have local knowledge and experience.
With IDX searches, even the MLS provides marketing exposure for the listing agent. Expand that to real estate websites that display the agent's name and/or contact information when the listing is syndicated, and this is a wide reach.
There are a couple of mis-statements sometimes made by List to Last champions though. In one article the author gave a "personal website" as one of the ways listing agents have over buyer agents in marketing themselves, and also stated that "buyer reps get almost no marketing opportunities for themselves."
Any broker or agent who doesn't have their own website or blog by now shouldn't complain about how much business they get. Buyer agents have very high exposure websites and blogs that bring them business. I practice buyer brokerage only in my area, and my blog brings me all of my business. Buyers can get my perspective on the market as related to buyer concerns. I don't have to balance my topics and post material to appeal to buyers and sellers. I can cover my market exclusively from a buyer perspective.
If a buyer agent wants a yard sign, there's usually nothing stopping them. You've just sold a home to a happy buyer, so why not ask to place a "This Home Sold By ___________" sign there for a couple of weeks? While it's not something I want to do, it is an option. Signs and listing marketing carry a price tag. Sellers still expect print exposure, even when it's dubious if it brings any value to them at all, and it's costly marketing.
In today's Internet world, a website/blog is all that a good buyer agent needs for marketing if they build and operate it properly. You don't need to list to last.