When someone pays down their mortgage, they lose one of their biggest tax write-offs.  That alone is reason enough to never do so. Additionally, if the investor has the intention to buy more real estate, making higher payments in an effort to reduce principal, simply reduces the amount of the capital available to buy more real estate.

Paying down a mortgage creates a significant amount of equity.  Equity, contrary to popular belief is not something someone wants.  Despite that we have been taught that it is a wise to keep equity in our home, it is not.  Equity is not safe, it has no rate of return, and it's not liquid.  In other words, equity doesn't help if someone's credit scores dive; they lose their job or get sick, because the bank won't lend them back their own money (their equity).  Therefore, their equity is not safe from life's unforeseen circumstances-not good for someone planning on using it for retirement.  Because equity has absolutely no rate of return and is not liquid, it has no power, accept to the bank that is using those payments to make money for themselves. 

The argument typically is that if someone doesn't pull the equity out in the form of a second mortgage or line of credit, they don't have to pay interest on it- a reasonable point.  However, it's fairly easy to take the equity out and place it in an investment vehicle that will return the rate at which the money is borrowed, thus washing it out.  In doing so, the borrower can take out their equity, not pay an interest rate, and have it accessible if they need to use it.