I haven't used the Explorers yet but other (plastic) H&B boxes.
In my opinion the advantage (or disadvantage, depends on how you see it) is that the boxes come off in a crash, not only the plastic ones, it is related to the attachment of the boxes and that seems to be the same with all H&B boxes (I have also seen the H&B Alu Standard panniers coming off in minor crashes).
The good thing is that the boxes won't break easily, not even the plastic ones, because they don't absorb the whole shock when they come off. Downside is they won't protect your legs. But then they won't injure your leg either by trapping it.
Matter or preference really...
PS: They seem to stay on in falls or (very) low speed tumbles, so they can't be nicked by simply pushing the bike over.