Fenders, and a properly fitting seat-post that has some grease on it will prevent most water in the BB issue

Of course holes are important, but, I am sorry to say, opening a slot in BB shell is about as dumb idea as they come. The fact this has been routinely done does not make it any smarter. I cannot think of any other part of bike more battered by weather than the BB area. Any sort of potential gains in moving water out faster should be completely wiped out by mud and salt coming in from below and getting stuck inside. In fact, you might start wishing for more rainwater coming down from above, to wash that out.

In any case, it is each person's choice what to favor. I rely on the standard holes at extremities + hole in BB with a screw. The screw does not close off the hole completely, even by mismatch of curvatures. The frame is steel. I have never hanged it from anywhere in any fashion, nor likely will. The bike stands every whole day outside, with no cover, irrespectively of weather and, to the best of my knowledge, the frame after 12 years is in a pretty pristine shape.

How well a small unshielded hole at BB bottom would work would depend, in my opinion, on the conditions one tends to ride in. I am sure that details of what is done to the BB do not matter if one rides in fair weather, running, on rare occasions, into a rain. From my observations, this is the type of riding done by 95+% of people. I am not in this category and, for the reasons indicated above, opening the BB will not be hanging around my mind.

Don't see any reasons against drilling a hole in the BB, wont be visable while riding, don't need to go for giant slots, just one little hole, I can't even imagine having to take my seatpost out and hang my bike upside down ever time I ride, what a hasstle, although I will say I live in a very rainy area (The city of roses and rain) and I never get standing water in my frame (even without a hole), dunno why but even daily commuting in the rain and I have never had a problem, maybe just cause I tend to overzealously overhaul all loose bearings.

I'm with you, I've never had standing water in my old Schwinn World Traveller and I even it rode it on the edge of the ocean getting sea water all over it, and left it setting outside in the rain...but after 30 years of this sort of abuse there was some rust inside the (sealed) BB area but not as bad as I thought it would have been.

This hole or no hole business is just going to have to be a personal choice. If you live in a dry climate I don't see why you would have to; but there are plenty of these enclosed BB shells running around in wet climates and they don't have problems either just as the slotted ones don't have problems. I chose to put a hole in my Trek because living along the coast condensation was a big concern and I thought it would be better to have a way for that moisture to get out. Condensation will form inside a frame even though rain may not be getting in.