Knowing Asian exams, you have my sympathies.
But that's potentially very useful. You could prevent anyone from forcibly Investing you, and the WoB mentions it could be used for other impurities, IIRC. So you could survive poisoning, being Lashed, maybe being Soothed/Rioted, and maybe you might be able to heal yourself if said impurities extend to disease.
I wouldn't say it's useless, just extremely hard to train to an extent where you'd be able to use it.
I can think of a few:
- Allomantic/Feruchemical pewter
- Feruchemical steel (a Scadrian version of the Troll Warlord in DotA 2 )
- Feruchemical gold
But the absolute scariest would have to be Allomantic atium.