I don't see any reasons why this suggestion should be implemented but I'll try to take one server experience and put on table pros and cons myself:
Pros:
• Racing alone is more efficient, no one is distracting or causing fps drops.
Cons:
• This could encourage usage of cheats as racing in own world makes you invisible (besides /spec function), other racers couldn't notice anything.
• Less racers leave at default world (probably all newbies play here) which removes mass effect from the race. Racing with many players is more fun, exciting and generally makes race more competitive. When pro racers (most likely) join own worlds, races will lose their spirit and become more like minigame for breaking records. Newbies could leave server and look for places where way more players join races.
I tend to stay neutral, perhaps lean to -1 because of mentioned second cons.