Gaskets are replaced upon reassembly because they crush upon the first assembly. If there were coolant going through that gasket, replacement would certainly be required to prevent leaks.

If you have the turbo motor, air leaking in the manifold may confuse the MAF, causing a MIL. May not though. Anyway, the air leak is probably small to nonexistent given the pressure involved.

Actually, I think you're on to something. I recommend using grease on the gasket when assembling to ease disassembly. If it doesn't tear when you take it apart, use it again if you feel up to it but grease it again. Note that grease has been used on gaskets since their inception. It swells the fibers and helps get a good seal with no bonding.