why is it private anyway?
RTB's failbin was public
Because initially it was just used for what were just considered stuffty, glitchy, ugly mods, and it wouldn't really matter if you went in there and downloaded it.
But then we started having more Add-Ons that were just terrible all around, particularly script-wise, and they could break servers or other Add-Ons. So they got CRC'd, and Badspot didn't want people having access to them, incase they got around the CRC or tried to fix the file and made it worse.
In addition to that, since there were so many broken Add-Ons in the failbin they caused problems for a lot of players who downloaded them but didn't understand where their issue was coming from.
So hiding the failbin meant fewer people coming to Help with problems caused by failbinned Add-Ons.
Generally speaking, if you're going to have a place where you put all your waste and refuse, it's wise to try and keep the children out of it.