Some of the affected lenses do fall apart, like my 105 DC. Others expose a very ugly opening straight into the internals of the lens, through which dirt or stray light may enter.
The design of the A/M was introduced in the early stage of the AF era and Nikon, for some inscrutable reason, probably thought the selector would be set to 'A' all the time ? Nobody really knows, though. And for reasons explained below, it really doesn't matter whether the ring was used.
The crack comes as a result of material fatigue, because the ring on some models simply is a little on the wide side (designated play to make altering the setting easier?), and there is only a single screw that keeps it in position. Over time this leads to the infamous crack. In fact, the A/M ring will develop a fracture even in the case it never has been used, that happened with my 200 AFS Micro after 11 years of non-use.
Some time during the early 00's, the design was altered and never seen on newer models.