Around here you need it for Nine Months of the year. Usually if they don't have A/C with the system they shut them down for the summer (3 months) Heat rises & with the heat system in the basement, in my opinion, should also be used to heat that space. I have had to open up a heating duct in customers house because in an energy audit that they had done the company closed the heat vent. They complained that there houses were colder than ever & their floors were cold.
I myself have owned a home that a former owner had everything insulated in the basement, the floor joists & hw boiler piping,etc & you could see your breath down there in the winter. You could see where he scorched joists trying to unthaw pipes. I removed all the insulation & insulated the peremiter better & never had a problem after that & my floors were warm.
This past winter & installed a pellet stove in my "new to me" house & shut the furnace down & our floors are cold, so I have to run an electric heater down there - you are not heating the space for nothing-as heat rises-and if your heating system is down there-in my opinion-use it starting there-you will be warm & happy

Mike