This is a reasonable question, though you could readily extend it to ask: why bother with the entire ship-sharing thing in the first place...?
I can see a use case, say I am a combat pilot and have all my ships equipped for combat, but suddenly there is a need for mining, say, tritium for jumping and I would like to help. Being able to borrow a properly outfitted mining ship, even if not engineered, would be useful.