Usually, I play one race. After being promoted to 1 more higher league I've tried to play random... Off course, I began to lose a lot... definitely, other races require other BO, other micro, other strategy, other skills.
What I see beneficial of playing random is:
Your opponent doesn't know your race in the beginning and can't choose good BO from the very beginning, he needs to scout earlier (hart economy) or start from 'common' BO that can't be too scary for you usually as it could be if he plays SPECIFICALLY against your race.
Playing (and winning) random you will get almost twice more achievements: for example for 100 win as terran you will get only 4 "Solo Terran 10, 25, 50, 100" achievements. While playing random you will receive 4 "Solo Random 10, 25, 50, 100" and 6 more: "Solo Terran 10, 25", "Solo Zerg 10, 25", "Solo Protoss 10, 25" (some variations could happen though).
You practice different races, so your knowledge about their strong/weak sides could be better then if you play 1 race only.
As a drawback I could say that you won't play your 1 race as good if you will play other races too.
What do you think about? Does other races knowledge value is bigger then better knowledge of your "own" race?
Thanks a lot.