Having the stones in your inventory does do something. It protects your party from the radiation in the Underdark. Leucrotta's lair is not a part of the Underdark, so it doesn't matter yet, but once you enter the mines, you are in drow territory and without those gems your party will die, once you try to rest.

Despite being named gems of infravision, those gems do nothing for your ability to see. You still have to cast a new light spell, when the old one runs out of juice.