Do I understand correctly that you can't pit and save the race state to disk, then go on to other events and come back later? If it's saved, I don't understand the limitation.
Yes that is how it works. It's most likely limited like that because:
A ) It took them a long time to get this implemented and probably took them a long time to get it to work [mostly] properly. The less variability when adding something difficult, the better. Doing other events would mean changing money, experience, items, cars, current car, car settings, miles driven, races won, car switch count, events completed/best results, best lap times, replays, photos and who knows how many other things after that race was saved. Then there is the problem of what to do with the car locked up in the middle of the race. Even arcade and online racing change things permanently.
Some of those things might seem simple to deal with, some would obviously raise issues, but
all of them are potiential problems that could very well cause untold consequences right away or some unkown time later. So it would be necessary to extensively test and be sure that there is no conflict anywhere with anything, making adding the mid-race save much more difficult and time-consuming. Many people recently had a problem after a game update that rendered the game completely unplayable until they corrected the [often multiple] issues themselves or waited until PD was able to patch the problem several weeks later. It's not very hard to screw up a complicated program. GT5 has enough trouble keeping its savegame from corrupting as it is.
B ) You're in the middle of a race. It makes absolutely no sense to stop the race, get out of the car, get in another car, go take some pictures, get in another car, tune it up, go run 4 or 5 races, buy 2 more cars, paint one of them, take some more pictures and then go back and continue your race where you left off. It would be nice if you could perhaps do some arcade races or an online race you were signed up for well in advance, but both of those things can affect your garage and car status among other things as mentioned above, and online racing even involves continuing your GT-Life "career" as it is accessed from the same game mode as the race that is saved. While Arcade mode is separate, it still can make changes to the cars in your GT-Life garage.
All in all, it is a small price to pay for being able to run the longer races without having to leave the system precariously paused for days or weeks on end. It should really only be an issue with the longest races, as most of them are short enough that with a little planning they shouldn't ever take more than a couple days to complete. There are some people who seem to be only able to ever get an hour or so to play in a day (or to sit still for half an hour at a time) but do still want to run the marathon races, and thanks to mid-race saves they now can. Beats not being able to. Hopefully in the future the major issues can be all sorted out in the initial game's release.
Now that's a right proper wall of text answer to a two-sentence question.
