"We've already discussed this at length," she continues impatiently. "Disillusionments are too unsafe in busy places. If someone gets suspicious and randomly casts a Finite, we're fucked. Ricochets are just as dangerous. Also, we don't know if they put up security barriers at the entrances and exits of Diagon Alley. A permanently installed uncovering spell like the Thief's Downfall in Gringotts would be our downfall if we were disillusioned. The same goes for the Glamour Charms. The only alternative is Polyjuice Potion. Would you prefer that?"