I've always done this to save on maintenance costs but I never really checked to see if the cost of building the PDC outweighs the maintenance costs you save and how fast you recoup those mineral expenses with the savings in maintenance cost. So I just checked. While this can hardly be considered a thorough evaluation I took the maintenance costs of one of my 3000 Ton jump ships and compared that to the building cost of a PDC with 3000 tons of hangar space, a maintenance store and nothing else but the crew.
It turns out that my 3000 ton jump ship uses 25. 3 elements of minerals in maintenance a year and my PDC costs 350 minerals to build. It is true that you don't recoup the construction costs, but the maintenance savings pays back your resource construction costs in less than 14 years. You do spend alot more in Vendarite (225 of the 350 mineral total for my PDC) than you would in maintenance but, at least for me, the resource savings and repair facilities are worth putting my military ships in PDC hangars.