All moral issues aside for now:
From what I know (regardless whether it's a demo or a version you licensed),
sampling a VSTi is perfectly legal - in case it's NOT sample based. But all
this only applies in case you make the samples publically available.
If you only use them for your personal purposes, there's no law against it
at all.
Whether you feel good about sampling demos or not is quite another story.
I sampled quite a lot of VSTis (and still do), basically to save up CPU
power, demos included - but meanwhile I own them all. To me that's just
fair, especially in case I use those samples a lot.
FWIW, I just started a rather intensive sampling project, covering the
EVP88, EVD6, ES2 and partially the EVB3 - to have some of my most used
sounds available in Cubase as well. In this particular case I will have no
moral problems in sharing them at all. The instruments are not sample based
(which would make sharing patches a copright infringement) and I wouldn't
feel bad about it at all anyways.
- Sascha