But what good would it do to keep an empty fuel tank in orbit ?
Especially one that's covered in foam that's off-gassing, and shedding small bits and pieces over time. Also, never mind the fact that the tanks were built as thin and as lightweight as possible, so had they been pressurized and converted into living spaces, they would have provided little to no shielding against space debris or radiation. Also, never mind the fact that by definition they were almost empty after launch, so you would still need to haul up all the fittings, equipment, furniture, etc... that you would need to stick inside them.
The reality is that other than the strong-back that held the shuttle and the SRBs together, the ETs were really just giant aluminum cans, and were about as useful as a spent aluminum can.