I'm not aware of myself having any phobia. I can imagine that I might not like REALLY tight spaces. Like when you see some spelunkers wriggle through a tiny hole or narrow crevice to get into a cavern on the other side. But I've crawled under things and been in small spaces without ever freaking out.
I think spiders and snakes are interesting. I've touched/held snakes and a caiman (the reptilian). I've nearly touched some spiders or watched them really closely, but I'm also not likely to hold or play with a large ugly spider.
I've jumped out of a perfectly good plane, and climbed trees and been on top of buildings, but I'm not likely to stand on the ledge at the top of a tall building.
Things that I think I would do.
I think I could/would do this if I needed to.
I am pretty sure I would not do this.
I would LOVE to do this
I would probably not push it to the extreme
I think I could manage a box of snakes.
I could not, would not manage my head in a box of tarantulas.
I'm with Glen, I figure I'm an 8.5-9.0. I figure Mike Rowe is a 9.0 - 9.5. To be a 10.0, you have to throw caution and common sense to the wind and be willing to do anything. The folks that climb crazy tall buildings and hang off of them, those are probably a 10.0 (although they may be able to hang from a 50 story antenna tower by their fingertips but be deathly afraid of spiders).
I watched a documentary on Jeb Corliss. He has always been "fearless" and when he was younger was suicidal, but IIRC, when he realized that he could do extreme daredevil stuff, that took away his suicidal feelings. It's like he has a need to do extreme stuff. And he says that he's not remotely self-destructive, because he's got lots of experience and each stunt that he does is meticulously planned to make it as safe as possible (because he doesn't want to die).
My guess is that if you find someone that's a 10.0, that they may have a psychological need.