Unfortunately, there is no one right answer to paste behavior on X11, unlike on native Windows and Mac implementations where there is essentially system-wide consistent paste behavior enforced by the underlying operating system.  As the Tk Wiki points out (and the comments hers concur), different users are going to have different expectations.  If we want to make this behavior available with X11 Tk, then it really should be a user configurable option.  Without providing an option, the overriding design principle is that the status quo wins a stalemate (http://www.boredomandlaziness.org/2011/02/status-quo-wins-stalemate.html).