tsig and servers help

[Terry]

Hello,

I am a bit confused about what exactly applying a key to the servers
part of the config does:

server 10.25.1.11 {
	keys {
		omajelns01.omajelns02;
		};
	};
key omajelns01.omajelns02 {
	algorithm hmac-md5;
	secret "asdfasdfasdfasdfasdf";
	};
zone "narf.com" {
	type master;
	file "/var/named/narf.com.hosts";
	also-notify {
		10.25.1.11;
		};
	notify yes;
	};

In this config, what does the key in the server section actually do
for me?  I really only want zone transfers to be secured between my
master and slaves but I don't really want each zone to have it's own
key.  There's also a very high chance that I will have to exchange
zones with non-bind servers too.  Which I don't think is an issue if I
apply keys at the server level.  Thoughts?

Thanks!