There are several dialogs where it would be nice to be able to copy the text of a line item so I can paste it somewhere else. For instance:
Add Addresses to the Rule dialog: any address in the Addresses list.
Settings → Mail → Blacklist and whitelist → Manage Blacklist dialog: any address in the Addresses list.
The reason I want to do this is so I can easily create a new custom rule without having to retype the address (I make lots of typos).
There are probably some other dialogs where this would be helpful.
By copy I mean drag the mouse over the text and hit Cntl-C.