My race completion is 100% so far.
I mostly do showdown races with D class cars, though, so I’m really far ahead up until the very end, which means that rammers can’t ram me until I’m, like, two yards away from the finish line. Therefore, I have no reason to quit.
Also, the Transit is awesome for showdown races.
You can report them to xbox live for quitting early. If they’re only doing it to answer the door or something, then nothing will happen to them. But if, they’re doing it all the time they will be dealt with. It’s classed as unsportmanlike behaviour.
I’ve only quit one race… and that was last night… not sure if anyone else has had this happen to them…
I was racing in a road trip… not sure what track… but I was in 2nd place… and I got a little sideways… hit a rock wall and was suddenly I was teleported into a screen saying i missed a checkpoint - but i hadn’t… then the screen kept flashing what appeared to be a lightened pic of the night sky… the counter would count down from 3 to 0… and stayed in a continual loop… I couldn’t do anything… so I had to reboot… kinda sucked since it was the last race of the trip… so i ended up getting nothing for the final standings… 
I’d rather them quit then wreck me. XP is still XP.
I never quit, last place or not - I did quit out of a roadtrip because I was sick of a viper “take out the Kid” (I’m 43 Moron, and if you spent half the time/effort you did try to wreck me into trying to drive, you might finish a bit higher…)
They were upset I got away so they couldn’t wreck me (it was a group) Scoobie S class is a bit to fast off the line for a Viper)
User was blocked
I was in an online road trip with 4 races in A class. I used my Ford GT. Some guys were just too fast for me on certain courses (the off-road ones), and others I held my own. I was running top three in one circuit race until one guy that was going so slow he was a lap down tried to take me out when I overtook him. I wound up finishing 5th place in that race, but I had so many Clean Race bonuses I scored the most XP overall in the championship and got the most Credits for it. I’d call that poetic justice.