So, I finally broke down and read this thread. Man, that took a long time!
Now my head is spinning!
What I've learned is that booking a room and not staying in it is considered unethical by some. Others feel it's fine as long as you don't cancel the reservation and keep the perks associated with the cancelled reservation. But there's a third school of thought maintaining that it is actually
more considerate to cancel if you're not actually sleeping in the room (especially campground reservations) so that others are not prevented from reserving that space. Apparently, it's also selfish to plan a 1-night camping trip and actually stay there, because those who camp for a week still get blocked from using that site.
In terms of the actual mechanics of the system and what perks you get with a throwaway room, it appears that you will definitely get free parking for the length of your reservation and possibly for the entire length of your tickets (that last one confuses me because even if you ARE staying onsite, you only get free parking for the dates of your actual reservation).
Having the throwaway room means that you get to book FP+ at 60 days instead of 30 days (that part I understand). Tell me if I've got this next part correct or not. Because FP+ is linked to your TICKETS and not room reservation, you get 10 days worth of FP+ if you have 10-day tickets (whether staying off-site or on-site). So, the only difference I see with that is being able to make the FP+ reservations 30 days earlier (which is admittedly a big deal) if you have ANY on-site reservation (even a 1-night reservation that may or may not be a "throwaway" room). But it's not getting you any extra FPs.
But here's the rub... then I read posts about having multiple sets of MBs and using those to get same day FP+ first thing in the morning (unrelated to the three you have pre-reserved). So now I'm thinking that even ON-site guests are booking an additional throwaway room (and maybe cancelling the reservation) to get extra MBs which ARE getting them more FP+ (albeit not more pre-reserved FP+... only what they can get on the morning of). But these extra MBs may or may not have to be associated with the same or a different MDE account or maybe not associated with any MDE account.
Man, oh man. I'm glad they replaced all of the "secret handshakes" * of legacy FP and replaced it with such a simple, straightforward system.
* disclaimer: The only "secret" about legacy FP was that you could use it even after the printed time on the slip. Well, that and the "secret" fact that arriving early = FPs available.