Asking during the debate puts it on MSM where it can't be ignored. But it also puts Obama in the position where he's unlikely to lie about it since he's wearing it on national TV, and he doesn't know what pictures people have already.
Having Mitt ask during a debate is 100% certain to produce wall to wall “Mitt is cruel/stupid/distracting/etc.” coverage of just that by the MSM and would risk his gains from the first debate. Not a risk I’d care to take. Commenting on how both are good family men who always wear their wedding rings and segueing into a story about his and Ann’s rings wouldn’t do that, and depending on the details might even win him a couple mindless voters. But it would encourage some mindless Obama worshiper to put out a corresponding ‘me too’ story on the Obama rings. Which then opens the doors for commentary on that in a way the current story (by Corsi in WND, so automatically tagged and ignored as ‘birther nonsense’ by MSM) never could.