The more I think about this the more I believe that GT (Sport) is no longer a game but even more of a thinly veiled talent spotting tool than GT6... that we've (again) paid for!
Daily races only give you a brief snapshot of ability before everything is reset. This actually benefits the more casual, less alien, racers but doesn't really let PD drill down into who's really the best of the best.
Daily races stop all people having too much practice; you jump in, like everyone else; (hopefully) have a blast, like everyone else; feel good / bad about your abilities based on how you've done against your own perceived skill level, like everyone else; I've you feel you did well, then great, if not, not to worry, there is a new challenge awaiting tomorrow.
Weekly races encourage those with more time on their hands to do lap-after-lap, day-after-day, constantly eeking out tenths or hundredths, pushing "normal" drivers, who can only jump in for 1 or 2 hours once, or maybe twice, a week, further and further and further down the rankings. Yes, it may give casual racers, who see a track/class/car combo they like on Monday and think "great, I've got the day off on Wednesday, I can do it then", but by Wednesday aliens, with more time, will be dominating the rankings and unless you're an alien yourself you probably won't be achieving anywhere near the rankings you previously got. You'll be then stuck with that disheartening and discouraging feeling that you're not as good as you thought you were, and less likely to play Sport Mode later in the week if you get chance.
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.