To buy airplanes to keep them sitting on the tarmac is money thrown out of the window. OK, an unscheduled aircraft does not need pilots and stewardesses, but the rest of your company is somewhat dependant on aircraft number, so you loose real money besides the theoretical interest you gave away by financing the purchase, and additionally lacking amortization of investions. Using spare aircraft for checks is only advisable for C- and D-checks for a fleet, that can keep an aircraft busy throughout the year flying substitutes.But to make that work you will have to plan to have one plane at a time in the hangar.
Spare aircraft covering A- and B- checks is nonsense. I am really interested, how you are planning around those spare aircraft.