Platinum is a noble metal used in real life for its anti-corrosive properties and is highly resistant to corrosion.
Platinum in the module does not protect against corrosion, however.
I think, in addition to electrum, it should grant corrosion resistance against puddings.
I do agree and pointed it out a while ago. The answes is that it was for balance issues.
To be honest, gold also should bring that.
If I were to take into consideration balance issues, I'd say:
Keep electrum as it is: +1 vc shapechangers, 5/- against acid, corrosion resistant.
Gold: influence bonus, 10/- against acid, corrosion resistant.
Platinum: I'd keep how it is: +2 vs undead is STRONG.