In my not so humble opinion, this is a very short sighted, ill considered plan by PD. Yes some people will go out and buy the DLC but I don't think it will be very many. For sure though, it's going to interrupt the flow of online racing. It's tough enough to find a good room to race in, now if they go to Spa or Kart Space or future DLC tracks, you'll no longer be able to participate without purchasing the DLC and many will exit and find another room. The more DLC tracks you get, I think the less online participation you will get because guys who don't want to pay for the DLC and according to sales figures that's about 6/7 original purchasers of the game, will give up online racing because they'll be leaving and quitting the game constantly.
For the few thousand they'll force to go and get the DLC tracks now, there will be thousands more driven away from the online portion of the game, and for me, this is not good for their future. I paid for it on Day 1, and I have no objection to sharing it online with someone who hasn't bought it. Remember too, PD took that option away from you, sharing the track with your friends, that you paid for.
Still it seems like a logical decision to only let people who've actually paid for it use it. I think PD had this option to help the people who couldn't use the DLC on multiple accounts and that's fixed now. Or they just wanted people to try the amazing track and suddenly not allow them on it anymore hoping they would buy it 
