The reason slotted shells have departed into the past is because AL and CF frames don't need them; there are still custom built French and Italian steel done this way. You need a drain hole for water to escape and even condensation over time can be damaging and thus needs to be drained. If the slots and holes were not important then why remove the seatpost and hang the bike upside down to drain? sounds kind of silly now doesn't it? When a simple hole could drain the water automatically without a thought or effort on your part. I've had the same bottom bracket for 30 years and over 150,000 miles with the hole in the shell...of course routine maintenance of the bb is very critical for long life.