Why no rich-text cutting and pasting between Linux apps?

I was wondering whether there were any reasons _in principle_ why Linux desktop environments should only allow for cutting and pasting between applications in plain text. After all, Windows and Mac users have long been able to paste in rich-text and images wherever the recipient application is able to support respective mime-types. Is this purely a case of the current toolkits' inadequacy, or does the problem go deeper into X11R6 standards? Michael