Weekly races just give the 1% of aliens more chances to battle it out and constantly try to best each other, this only benefits PD for their GT academy. When you do encounter alien(s), who've had more time to practice, you, as a casual jump-in-once-a-week gamer are not going to beat them under any circumstances unless they make a catastrophic error.
Again, for the reasons given above, your relative rankings will be worse as there is always going to be aliens who can put in far more time than you do and utterly dominate rankings for weeks at a time. You may get a little more time / chance to race on combos you like but those combos will come round far less often.
So, for the average gamer, lower rankings and less combos. As I said, this is only really designed to benefit PD.