i both bf and pump for my twins and have never had a supply issue and usually have a large freezer stash so it never was on my mind to have back up for my back up. but then a few weeks ago i had a close death in the family and we traveled out of state to be there and so much sadness and stress and my really abundant supply dwindled to nearly nothing for a few days.
i was frantic as i struggled to get enough out of me for my babies. i've never been so stressed about anything with the babies as i was last week. finally i went out and got some formula that i felt was decent so i would not be in a position to have to rush out in the middle of the night and get whatever i could find while they were crying hungry.
thankfully it never came to using it, i got over the hump and things are on the good upswing again now. but that was a fear i never want to have again, we are keeping the bottle i bought in the car so we always have it.
after reading this thread i think i will get some more and pack it with the other emergency supplies, because when push comes to shove, frozen BM does not keep without a freezer! and clean water is not guaranteed, so ready to drink formula is a must for our piece of mind