Strictly forbidden? No.
But we already strongly encourage users to quote/include the relevant information when answering a question, and to include a link out to original source material. That's the best of both worlds; useful information with proper source attribution for reference by future readers.
I disagree. The answers most people provide simply graze the surface of the technology in question. Providing an external reference saves SE from becoming a dumping ground for useless/specialized information, but still makes it a high quality problem/solution site.