Last January both DH and I had problems at the front gate with our MBs - DDs read her ticket. He had his hard ticket and just used that to enter, I only had a photocopy of mine, so I was made to leave the line and go stand in the long line at Guest Services, missing the park opening we had worked so hard to get there for. In Nov, DSinL and I each had a park day where our bands did not read at park entry. We used our hard tickets with no prob and went to GS later in the day to have them fix it. I will NEVER risk park entry depending on just my MB - this system remains very glitchy and if you are the 1 or 2 (or 100 or 2000) people who get caught in their glitches, you will be glad you carried a back up.
And I don't distrust all technology, but, seriously, the inconsistency of Disney IT has baffled me for many, many years. I just made a DVC reservation for AKV in Oct and MDEx is listing my check in as March 21st (instead of Oct 1) and says I'm staying for 199 days!! I wish! LOL The DVC site has the reservation listed with the proper dates, so it's just another MDEx glitch - that I have to get back on the phone for and have corrected.