Damn, deepbit's down again. I definitely vote for this solution: switch to backup after two-three (or more) periods of idling. For example, if I have "Restart after an idle time of 30 seconds" option, switching will be executed after 1+ minutes of idle time.
If you don't like the idea, then I definitely vote for button that forces a client to switch from primary to backup or backup to primary, it was mentioned before by smackdaddy.
That's just too sad to lose 10 minutes of mining when you can lose only one