Yes for the most part investors are buying duplex/triplex/fourplex properties for income. However, what they are willing to pay is seen in the PRICES of these properties, no matter if income approach supports a higher or lower figure. Whether we agree or not, res lending , (of which financing 1-4 units is part of, ) wants to see SALES prices more than income . This is because their interest is in the property as collateral they may have to sell one day rather than how much it generates from rents, since lenders are not in the property rental business.
A lender is our client, not the investor buyer. To an investor buyer, the income might be more important. To a lender, the sales prices are more important. The lender may have to sell the property one day in case of a default and they won't be renting out as an owner. That is why they want more reliance on the sales comparison approach.