What is the logic behind using just one cost of debt financing rather than estimating the cost of financing with each specific issue of long and short-term debt?

What will be an ideal response?

The logic behind using just one rate–a long-term yield–for both short-term and long-term debt is that while short-term yields may differ from long-term yields (usually lower), over a long period, short-term rates on average tend to be similar to long-term rates. This logic is consistent with the "unbiased expectations theory"