I know that there are some people on this thread (including myself) who have posted that they've done RotR more than once, however, I really don't think this constitutes a large group of people, i.e., enough people that the few of us who've done it (I rode twice--Dec 13 and 14) would have had a great impact on many other people's ability to get a BG. Of course, I have no stats for this. If there were thousands of people every day at DHS who had already done RotR and were trying to get another BG, I might agree with you about limiting the ability to get a BG in the way that you suggest.
I guess I feel it's already limited. You can ride only once a day. If you're on vacation and you want to ride RotR every day, why should you be stopped from at least trying? It's your vacation. Granted, it's someone else's--many some else's--vacation, too. But on that note, perhaps anyone who comes to WDW more than once every five years should have their FP+s for major attractions limited, since they're "preventing" someone else from getting a FP+ for an attraction they may have the ability to ride only once ever. Or, say, give up their ADR for a popular restaurant since they've already been there once and they're filling up a reservation that could be had by someone else who's never eaten there. Or stayed at a resort, etc., etc.
RotR is brand-new. They're still working out the kinks. When I was there on Dec 13 and 14, there were a lot of people queued up outside the entrance very early. There were no backup BGs then and, I think, not a host of bitter complaints by people who couldn't get to ride. Also, it took longer for the BGs to fill up. Were there fewer people there those days? Maybe. I have no way to gauge this. But it makes me wonder if the backup BGs are such a great idea since you may not get to ride if you get one of these. At that point, you have entered a lottery. For me, I'd rather get to the park, get or not get a BG, and be done with it so then I can arrange the rest of my day accordingly.