If I recall correctly I did it because I didn't want to miss out on any mathsy things my friends would get to learn who did it. And because GMA we had a really fun class and everyone was really passionate about it.
Although last year, while the content was real interesting, class was a real struggle- I'm not sure if it was because our really enthusiastic yr 11 teacher left, or because our new one (really nice person, but hadn't taught the subject in 5 years apparently) wasn't as... I don't know how to put it tbh. I guess just being in a situation where if I didn't know the answer, there was no one to really go to in person (AN was great but it's a different type learning- I really don't know how zsteve does it..)- that kinda put me off continuing down a 'mathsy' career.
I guess it's 'potential' (just the idea that it can) to scale significantly above 50-not that it did-was also a contributing reason for me not dropping it after headstart (in which some of my classmates did). In that sense I chose it for similar reasons why one might do further.