I would be very surprised if it is that high now even with the early adopter skew. Reports say that last year it was around a quarter of smartphone users use passcode locks on their work phone (http://www.welivesecurity.com/2012/02/28/sizing-up-the-byod-...). I imagine 5S rates are higher than that, but 90% would be insanely impressive. When it comes to computer security, as usual, people's apathy is the biggest problem.

I'm sure opt-in/opt-out is a major factor, too. I don't have a 5S, but I'm pretty sure it's opt-out. I think even after upgrading to iOS7 I had to opt-in again to turn on the numerical pass code.

iOS setup flow basically points the user in the direction of setting it up.

Isn't passcode required to get exchange email on iOS?

That's an option set by your IT department. Annoyingly, mine does the same thing. It doesn't have to be that way.

At my work they have allowed the TouchID to be used with our security policy. I just haven't shelled out the money to buy a new phone.