Well, for online check-in, it's way faster. I can do it while waiting for a crosswalk rather than having to think about it when at a PC. But, I often go and collect a physical one from the check-in counter for the reasons cow mentioned. With my #3 reason being it's easier to prove if AP miles are missing..

Regardless on whether you use it or not, if they push out a feature it should have undergone at least minimal testing. This one is an obvious fail.