The logic being: the more people taking the exam (mathematics or other subjects), the more likely they will have extra materials.
This might fire back in some areas. The biggest test center here ran out of seats well before the registration deadline. If I had to try my luck as a standby, I would try one of the smaller centers who accepted registrations right up to the deadline. They may or may not have extra booklets, but at least they have seats.
Glad that it worked out for you!
Yup. The reason the proctor didn't want to take standby's is because there were so many registered test takers (i.e. they were overbooked). All test takers (across subjects) were in the same lecture hall, and apparently they didn't have extra seats so the proctor just didn't want to deal with us. Once everyone was seated though, the proctor saw that there were plenty of no-shows, so she had no choice but to take us in.
On the other hand, I've heard of at least one case where a comp-sci test-taker went as a standby but had to leave because no one registered for the CS test at that location, so ETS didn't mail CS booklets at all.
I really don't know how they handle extra booklets -- both times I took the math test there was at least one extra packet (which includes like 5 booklets or so?). I wonder if extras are still sent when just 1 or 2 people are registered for a given subject.
Either way, if you go as a standby, nothing is guaranteed. Some people are lucky, others aren't. My guess is that the majority of standby's are able to take it, though.