Open bands should be checked every few months to make sure they are closed tightly.Over time the gap in between the band can slowly get larger allowing a bird to maybe get caught on a cage wire and not be able to get lose.
Another reason I like open bands is that they can be removed.
From my experience over the years I have found that most of my customers just simply don't like the bands on the birds.
Bands are also an inexpensive means of identification.
LOST OR STOLEN BIRDS Bands are one method of identifying a lost or stolen bird.
If i were using closed bands I would have to make sure that I put the bands on the babies at exactly the right time otherwise it is to late and the bird can not be banded.