The "why" for using an acid flux is easy. It works better than rosin. But as you found, rinsing, flushing, and oiling through a little weep hole drilled in the lower rib isn't likely to get it out or neutralize it.
I would like to find out if Oatey Lead Free Flux is acid based or not. It is the best stuff I have ever tried with 50-50 tin/lead solder, even on joints I could not clean as well as I'd like. It has very fine particles of tin suspended in the paste, and really does a nice job of tinning the surfaces by itself. I think it would be great for ribs if it is not corrosive.
