Public Key crypto was discovered by GCHQ (and then given to the NSA) several years before it was publicly discovered by Diffie-Hellman and RSA. I think this was to avoid having to have the symmetric keys under armed guard. The public discovery is also what kicked off the 'crypto wars'. I'd be surprised if modern nation state intelligence communities found symmetric encryption sufficient.
Oh yeah, I'd forgotten about Cocks. Blame my selective memory for withholding credit from people who don't share.
Spooks would of course welcome any discovery, and asymmetric crypto does solve problems for them (getting government crypto distributed as wide as possible). I am saying purely symmetric is "sufficient" for their core functionality - the communications that really need to be secret. Coupled with the head start before asymmetric was even discovered, that is where their focus is going to be.
Put another way: if you were in charge of securing communications and had to prioritize resources, would you rather research a trustworthy asymmetric algorithm or a trusty symmetric algorithm? Likewise if you wanted to snoop on others' communications, would you prioritize breaking symmetric or asymmetric techniques?
A lot of interesting information about the history I learned from Steven Levy's crypto: http://www.amazon.com/Crypto-Rebels-Government-Privacy-Digit...