I have the opinion that it should be worded in the context of E&R, and give information about what type of questions are on/off topic for that particular tag.
Indeed, you're absolutely right, this part is critically important.
I updated the sidebar editing help for the Tag Wikis to give better help on this. Let me know if that help can be improved; just bring up a tag wiki and check out the sidebar.
Is it acceptable for a user to copy and paste a load of information from Wikipedia into a tag wiki?
It can be so long as it is properly attributed and linked back to Wikipedia (in the full wiki, since excerpts don't allow links). But as you said --