I have a custon built frame with brazed on cable guides under the bottom bracket. At the time it seemed like a good idea, the builder certainly thought it was a step up from the basic moulded plastic cable guide.

As an Engineer, a steel cable running in plastic ((I assume it is nylon or similar) is a much better engineering solution than one running on steel. Add a paste of mashed paint and a bit of rust that will develop in the steel guides over time and they seem even less of a good idea.

Having said this I have had no problem with my custom frame, I just cannot see the point of brazed on steel cable guides under the bottom bracket.

Am I missing something?

If you are worried about rust there you can use a small piece of plastic cable tubing to protect the paint. Not sure why they didn't run the tubing on both cables here but think you will get the idea...

That is what I did on my frame but the sleeves move over time and end up in the wrong place and doing nothing.

Also from experience - in my younger days when I still went out in the cold - the tube can get water in, freeze and hamper gear changing.

I haven't got a problem with this art all, just wondered.

Helicopter tape?

over the BB cable guides were made in steel, long ago..

No you're not missing anything except to stress over a detail that's pretty minimal. I've done the brazed on cable tubes thing for a few decades and excepting once with a neglected galvanized cable and an electrolyte drink spillage have never had any issues with cables, corrosion or friction. However I, too, stress over tiny stuff (or why would I make my own frames) and will likely go to plastic bolt on guides in the future. Here's a shot of such a set up that I made in 1985. Andy

When I add brazed-on cable guides, they're either brass or stainless. Carved up to allow for any water to drain out, rust has never been a problem.