Slitty Slots

Why the hell does Shimano have open slotted cable openings in their STI levers? I can't think of why this might be an improvement over their older design of a closed-hole cable retainer. I've seen several bikes this year with frayed and sheared derailleur cables all at the cable head. It seems to me the slotted STI design is the culprit. Now, most of the folks I've dealt with that have had this problem are suspect as far as their mastery of nuanced shifting is concerned, but this is a problem I haven't dealt with in such numbers before. Anyone have a good reason Shimano does this? Anyone?