If the cable guides are on the top, your sweat can build and are attracted to the guidees through capillary action. You see lots of vintage bikes that are rusted there. Regular maintenance, of course, eliminates that.

Open cable stops on the bottom of the top tube are places for water intrusion into the cable housing. If you throw the bike over the your sholder to carry it you're grabbing onto the cable. But you have less housing to compress, and a nominally stiffer cable routing.

Then there's the "hidden" cable that you see on french constructeur and higher end Italian vintage bikes, as well as many or most carbon fiber bikes nowadays.

But, meh, not enough to worry about one way or another.