@Carlos Lopes mortgage paydown and tax savings are usually mentioned as benefits but rarely factored in the equation for three reasons:
1. they're variable from investor to investor. mortgages are always different (5-30yr terms, different interest rates, fixed/variable rate, interest only / fully amortizing, etc.) and everyone's tax situation is different
2. they're hard to calculate because they're "paper" gains. Mortgage paydown is great but you can't access the equity unless you sell or refinance. What if the country goes into a recession and your property loses 20% of it's value? Then, what happens to your mortgage paydown? Any equity "gained" is wiped away.
3. Cash-on-cash return is a simplified calculation and translates across most properties and investors
I always keep track or mortgage paydown and annual tax savings separately from my cash-on-cash calculation. I don't do anything with these numbers when I'm analyzing new deals, but it's good to know how much equity is built up or how much I'm saving on taxes.