With the small amount of time it takes to check (2 seconds) why are you concerned with "spending time" checking PH. (harsh comparison) That's like a meth cook saying "oh I have pure red phosphorus, and didnt have to use broken down cold medications and car batteries...... should I bother checking my mixture?"
While acid rain is uncommon, it is not uncommon for a nasty storm to whip up and pick up a bunch of dirty ass debris in the area.
Again. 2 seconds, and a meter is all it takes.
Notice how some days it rains and a car looks dirty. Some days the water drys clean as a whistle, somedays it looks like pure shit and you need a car wash. That means rainwater is very inconsistent.