There are. But if the IETF wants to encourage code that anybody can study
and/or borrow, GPL is not the way to go.
I don't think the "study" argument flies anymore. It's akin to banning
a doctor from reading "one flew over the cookoos nest".
With all the discussion about code, I'm much more interested in good APIs.
I think the advanced socket API for IPv6 is one that works really well.
More recently, getdns defines for the first time a sensible API for DNS.
(A bit verbose if you write C, but certainly at the right level of
abstraction).
For APIs, you'd like to have at least two compatible implementations of a
single API. So if one of those is GPL and the other BSD, there is no
licensing issue.
There may be other uses for code, for example to illustrate a complex
algorithm. But the one thing I miss most are well defined APIs.