How in the hell would fenders help the bottom bracket? BB gets wet regardless of fenders.

IMO fenders are for you, not the bike. Get them if you want them. The saying is, "Fenders...because that's not water." Meaning that the stuff splashing up on you from the road could be oil, could be pesticide runoff from the lawns, could be roadkill juice, could be wino urine, could be animal scat.