Well, I once wore a bullet-proof vest and it was raining. The vest got wet. No one shot me, but I'm pretty sure that if they had, it would have protected me. This is because rain is expressly not bullets.
Rain and bullets don't work on the same principles.
However, a bullet-proof vest and mage's robe (as you have it) don't seem to work the same way either.
A bullet proof vest works by absorbing the force and spreading it out.
But in your question you actually say the word "parry."
That is NOT the same principle as a bullet proof vest.
ward off (a weapon or attack) with a countermove.
Not the same as absorption. Basically, this means the magic pushes back and deflects against the source. That is also not the same as a force field. Magic that parries a blow is not a force field.
Here's why rain would not be repeled: because in situations other than battle, something that parries against ANY force, no matter how weak, would be...not tenable.
Someone brushes up against you in a crowd--your armor parries. It's not good and not practical, and it can also be a drain on the parry. If there's a finite amount of energy it can expel, pushing off everything from a leaf landing on you to rain is stupid. And it will run out.
I see a few exploits here.
A) IT PARRIES EVERYTHING (but it has only so much energy or energy fed by the mage!!!!!! If it does, I am definitely going to be pelting you with so much gravel, and water for as long as I can, anything that touches the whole surface and causes it to react. Let's not even talk about what might happen if you are made to go SWIMMING in it. The water pressure from every side? Could it be viewed as an attack? Would it be 'smart' enough not to? If it didn't, how impossible would it be to swim???
B) THERE'S A FORCE THRESHOLD FOR PARRIES. Rain, acid or not, is not enough to trigger it. So the enemy might be looking into acids that will damage the parries in places in order that their more ballistic attacks get through.
Look not upon option B as a stumbling block, but as an opportunity. How's it going to be when the seemingly untouchable hero (or villain) gets defeated by a clever person with water balloons and arrows. The first person who figures this out and manages to carry it out is a tactical genius.
Anyway, my take on the info given.