I think the Petit LeMans races would be very worthwhile. I would never do a 24 hour race in GT5 or 6 if they were added. But the shorter ones I would. The only thing that might make a longer race doable is the ability to postpone the race. That was added to GT5, but to have to come back and complete the race before doing any other racing is out of the question for me. I don't have the patience or the desire for it. I can see it being an option though for those that do like the challenges.
It would be nice to have them added and the ability to save the race and come back to it any time, still being able to do other races before coming back to it though.