I think the OP's question is an excellent one, I don't think there is ANY point to getting an XP award for the race when you are maxed out already. People seem to think he is questioning the point of the race, when in fact he is questioning the point of the XP award.
And to think, someone at PD had to actually calculate what the XP award "should" be for this race, and set a value for it, without actually realising that it is a pointless award.
Of course, if it is true, as I have heard elsewhere, that you get a LVL 24 gift car ticket for completing all the endurance races, then that alone would make it well worth the effort. There would only be three cars (F2007, F10, and FGT) that you could recieve. If you recieve the FGT (33.3% chance), then this would be the only way to own that car with zero miles on the odometre, brand-spanking new. Very rare, and worth any number of 24 hour races, IMO.