In other words, voting on the Rosary, millions use it, I reject it, but after reflection, it is millions and I am only one so I'm going to use it too. So it isn't a question of authority or efficacy, it is a variation of 'why everyone else is doing it', why not.
Remember we’re talking about an individual and not a topic for a debate team. After all, what is the implication of denying those who use the rosary. Is it not the default position of protestants that those who use the rosary are idolators raising up a dead human being above Christ through the use of pagan repetitive prayer? I suspect that it is a source of serious consideration for someone who denies millions and calls into question their ability to go to Heaven as a result of this practice. So instead of thinking of this as a, “so many are doing it, why aren’t I?” Question perhaps the question would be better phrased, “why do I condemn so many?” I think that is more in keeping with the spirit of the original question.