OK, I see - but is an Export/Import an easy process to set up?
How about implementing a feature like Merge Paste, ie overwrite tags which are represented in the paste buffer and leave the rest as they are?
Or better still: this could be the default behaviour for paste - you could still make sure that no old tags survived by deleting all destination tags first, then copy + paste?
EDIT 13. septermber 2011:
A few more questions:
Has Copy/Paste from Edit Menu the exact same functionality as right-click Tag Copy/Tag Paste?