Thanks for the replies so far, they are all very helpful.
3holelover, interesting point. I guess the simple answer is to monitor your good tank whilst crossfeeding. Sure there is a chance that you might start syphoning from the good tank, but proper monitoring should cover that. If however you do nothing you are guaranteed to loose a lot of good useable fuel. Which is the greater evil?
Avenger, the wing will empty anyway due to the leak. If wejust look at the 320 that is of no great consequence. Neither are dry running pumps, for which you get a warning.
So I guess it is safe to say that it all depends on the situation. A leak over Europe means you land asap without complicating the problem. A leak over the Atlantic, well...you better be sure what you are doing?