Better then going it wasted, you can use free HDDs for mining and they can earn enough money for you to pay some of your utility bills. If MASS values go up in future it can give you good amount of revenue also.
Yes if you are in such situation then you can invest your HDD in MASS mining. I don't know how much is return on MASS these days but something is better then nothing. If your HDD is going idle then invest that in MASS.