You raise a good point: it's always best to have a healthy amount of doubt whatever a doctor tells you. Question him/her to the point of being obnoxious. Then do your homework, A LOT OF HOMEWORK. If I was Ms. Jolie I would want a full accounting for the "87% chance" figure she was given. Since they gave such a precise number I would assume there is strong data/analysis behind it. However if it is bogus then, well, the situation is beyond scandalous.
Ultimately removal of female reproductive organs is not the key to breast/ovarian cancer prevention. Clearly diet/nutrition, plus regular screening, is the key. Yet there will be women who chose to have their ovaries/breasts removed. You can't get Breast Cancer w/o breasts. This provides a peace of mind for those women, like Ms. Jolie, who witness a family member suffer horribly from cancer.