I have used the same MB for three trips now, totaling 22 days and running in the half and full marathons. I have not had it fall off once. I check it every time I put on or take off my backpack as it tends to snag when doing things like that, but so far I have been fortunate.
I also take it off several times/day as I cannot sleep or shower with something on my wrist, so the connecting holes and pegs have seen some wear & tear, but still hold firmly.