So my kerberos knowledge is admittedly limited but we've had that talk a couple of times as well. Here's the concerns (the first is kind of unique)
1) No one works with the Fedora Project as their only job. Which means it's likely some people will have to register with two kerberos environments in order to do their day job and work on Fedora. My understanding is that's fairly complex and not all of our contributors are very technical.
2) If someone has my password and ssh key, doesn't kerberos not do anything to protect at that point? That's why we're thinking hardware key, but some of our admins are very opposed to it.
/me invites anyone interested to join the fedora-infrastructure-list and discuss this. It's a topic we take pretty seriously.