Withholding source code via cryptography, obfuscation, or just not revealing it for a period of time is both legitimate and moral. This is a critical component of expression, which is composed of both speech and silence.
Legally removing a modifier's right to silence effectively criminalizes a large number of possible business plans that would've benefited countless people.
One can exercise his right to silence by using GPL'ed works privately. The license provides for such use.
I'm talking about distributing what I want to others who express an interest, without being required to also share the source or meet some other requirement of a license.
Personally I don't see an incentive for someone to withhold code for a long period of time. But, I do see the potential for many business plans such as releasing binaries to an app store 30 days before releasing the code in the public domain. I think this type of arrangement could be the root of many an innovation in software distribution and sales, but it's hard to tell given the presence of both IP in general and the GPL in specific.