Cudaman its foolish not flush & bleed your brakes,It takes so little & the gaines are huge.
Maybe I’m missing something but how can fluid become dirty, contaminated and have moisture if the system is closed? Now if you have a line brake then I say maybe.
Brake fluid is highly hygroscopic moisture gets in regardless through the M/C lid seal & caliper/wheel cyl.seals the fluid becomes discolored & rust forms,the pedel feel can be night & day after flushing,try it I dare U.