when the bra is left on during a rain, it won't dry on the underside very well and may stay damp causing mildew. Not only will the bra be damaged but the paint may get spots as well.
Since you're in a rainy area, I suggest looking into getting a clear bra put on. If you search the forum for "clear bra" (including the quotes) you'll find several on my posts (w/pictures) on that topic.
2004 E60 530is, Delivered 12/12/03, TiSilver/LGrey, PP, SP,PS, PDC, ACC, FRS, Step, Xenons, clear bra, body colored rear inserts. (Retired, 1983 henna red 320is)
2004 Corvette Z06 Commemorative Edition