I haven't interpreted the WHO recommendations to be primarily for the wearer. Their statements have stated it's to minimize community spread, same as it has been. There is a lot we don't know about the transmissibility of the variants. We know vaccinated people are continuing to contract the virus (it's not an impenetrable shield), and the virus is replicating well enough for people to test positive. So what does that mean for transmission? The studies so far show that transmission by vaccinated people is greatly diminished, but that doesn't equal zero. Laws of large numbers, again. I've seen studies that indicate a one-to-one transmission between a vaccinated person is unlikely because their viral load is lower. What I would also like to see is what happens when several positive, but vaccinated people work in the same poorly ventilated space for 8 hours, 5 days a week. Can people cumulatively expel enough virus that lingers, that an unvaccinated person can still become infected with a more transmissible variant like Delta, and then go onto infect other unvaccinated people in a one-on-one, full viral load manner. I interpret the WHO's recommendation is that we can't wait for updated studies with every variant of what is safe or not. It's better to be safe than sorry and if everyone continues the easy precautions of hygiene and mask wearing then that will deter new infections from occurring for everyone, which translates into less community spread.
Unlike the US strategy which is challenge trials for everything since the unvaccinated will be the ones to realize the consequences.